two types of research studies often used by behavior geneticists

Behavioral Genetics | Definition, Types & Examples

Chris has over 20 years of experience in research analysis, program development, management, and adult education. Extensive global experience collaborating with diverse ethnic and cultural backgrounds achieving mutual goals. He has a Bachelor of Science in Sociology from Southern Utah University and a Master of Science in Adult, Occupational, and Continuing Education from Kansas State University. He is currently completing a second Master of Arts in Clinical Mental Health Counseling with Southern New Hampshire University. He also possesses a Diversity Equity and Inclusion in the Workplace Certification from the University of Southern Florida. He has served in the U.S. Army for over 28 years, retiring in 2023.

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Table of Contents

Behavioral genetics definition, types of studies in behavioral genetics, statistics in behavioral genetics, lesson summary, how do behavioral genetics explain behavior.

Behavioral genetics demonstrate that both genetics and environmental exposure can influence behavior. It proves what behaviors are heavily influenced by genetic traits and which can develop due to environmental exposure.

What do behavior geneticists do?

Behavioral genetics identify the relationship between genetic traits and environmental variables influencing specific behaviors and determine the strengths and weaknesses of these relationships. They focus research on twins and siblings with similar genetic characteristics and analyze ecological relationships and the similarities are differences in behaviors.

What are behavioral genetics?

Behavioral genetics is the study of the influence one's genetic makeup and lived experiences within their unique environment have on their current behavior. Many use the term nature versus nurture to determine if a particular behavior results from nurture, the environmental exposure, or result from nature, the genetic makeup.

The assumption that human behavior is influenced by genes and exposure to the environment in which one interacts is known as behavioral genetics . With its founding credited to the Englishman Sir Francis Galton in the late 19th Century, the field of behavioral genetics examines both genes and environmental exposure to uncover the root cause of certain behaviors. Throughout human development, lived experiences and exposure to the elements within the environment combined with existing genetic traits influence behavior. Behavioral genetics aims to understand how these existing traits, combined with ones lived experiences within their environment, influence specific behaviors. Behavioral genetics helps researchers uncover the root cause influencing certain behaviors.

Examples of Behavioral Genetics

• Researchers have discovered that the root cause of generalized anxiety disorder (GAD) is approximately 30% genetic and 70% influenced by past lived experiences. Researchers have also found that some with genetic traits of GAD may never develop anxiety due to the quality of their lived experiences within their environment. Twins separated at birth with the genetic characteristics of GAD may see one twin develop symptoms of GAD while the other may not be due to different environmental exposures.
• Empirical evidence suggests higher levels of mental disorders within the African American population. Researchers suggest this is due to either genetic traits or exposure to poverty, violence, systemic racism, and increased social stressors found within their environment. Adopted siblings placed in homes of differing social-economic statuses may develop different behavioral traits due to their environmental exposure.
• Researchers have discovered that the level of educational attainment is only one-sixth genetically passed down to offspring. The most significant factor influencing one's academic ability is their socio-environmental experiences.

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• 0:04 Twin Studies
• 2:00 Statistics in…
• 3:05 Family Studies
• 4:15 Adoption Studies
• 5:20 Lesson Summary

Behaviors, personalities, and psychopathologies are influenced by exposure to the environment or genetic makeup. Researchers have used family, twins, and adoption as data sets to analyze and quantify the environment's effect on behavior over the person's genetic makeup. Research has looked to answer if genetic traits have a more substantial influence on future behavior regardless of the environmental variables.

Twin Studies

Twin studies focus on the behaviors of twins raised in the same family environments and determine if the genetic or environmental variables have more influence on behavior. Variation exists in these studies. Researchers consider whether the twins are identical, also known as monozygotic sharing all of their genes, or fraternal, also known as dizygotic, only sharing 50 percent of them. Researchers analyze the variables of the environment and the unique genetic makeup of the twins to determine if they lived experiences and exposure to the variables within their environment have more influence on behavior or if the effect is due to genetics. There is an advantage of twins' studies and determining whether genetics or environment impacts behavior. This allows researchers to determine if certain behaviors result from genetic makeup or lived experiences within the environment. It enables medical providers to determine the best route for treating psychopathologies such as depression, anxiety, dyslexia, or neuroticism.

Neuroticism is an excellent example of an area where behavioral genetics have focused their efforts. Neuroticism is a personality trait where one cannot regulate the negative emotions that influence constant guilt, depression, and anxiety. When someone is diagnosed with neuroticism, they struggle with regular daily activities due to the inability to regulate the negative emotions associated with everyday stressors. Studies demonstrate that neuroticism is 40-60 percent heritable. Yet other studies indicate that childhood trauma also influences the development of neuroticism. Research would analyze the environmental lived experiences and the family history to determine where the significant influence lies.

Family Studies

Family studies require participants to have at least two siblings with similar genes, helping researchers determine the root cause of certain behaviors. Psychologists and behavior therapists gather information on family history during intake assessments to support the development of their treatment plan. Sometimes a behavior may be a genetic trait. Yet, the symptoms were never prevalent amongst the parents, and the patient may state on the assessment that there is no family history of the problem. For example, some can have the genetic traits for anxiety, yet their lived experiences never influenced the symptoms to the surface. When someone believes that their issues are due to genetic conditions and are the starting point of a genetic study, they are considered a proband. Typically, the proband has no past family evidence of a genetic disorder, and an individual is the first family member to undergo a genetic study. When geneticists look at behavioral traits, they will analyze the genetic characteristics of both first-degree and second-degree relatives. A first-degree relative shares close to half of the genetic markers. This includes siblings, offspring, and parents. Second-degree relatives share about 25% of genetic traits: grandparents, aunts, uncles, grandchildren, nieces, nephews, and half-siblings.

Adoption Studies

Adoption studies are another method of behavioral genetics. A key concept within the study of behavioral genetics that adoption studies provide is the concept of nurture versus nature . Or, in other words, environment versus genetics. Nurture describes the lived experiences and exposure to the variables within one's environment. For example, if a child grew up in an abusive home, they may develop psychological issues as an adult compared to a child in a healthy environment free of abuse. The first child's problems would be a result of poor nurture. Nature results from one's genetic makeup and is passed down from family members. For example, dyslexia is regarded as a genetic condition. A child can be raised in a healthy academic environment and still find difficulty reading and writing because of genetics or nature.

A well-known example of looking specifically at several cognitive traits, behaviors, and interests is found in a 1990 study published in the journal Science titled, "Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart." This study examined identical twins' behavior and cognitive traits separated and raised in two distinct environments and twins raised in the same environment. Researchers discovered that genetic markers have a more substantial influence on behavior than the child's environment. This means that identical twins raised apart will demonstrate similar characteristics as identical twins raised together. They even found that genetic factors could control the effects of these ecological variables under strong environmental influences.

In behavioral genetics, researchers use a statistical term called the concordance rate. Concordance rate is defined as "the percentage of pairs of twins or other blood relatives who exhibit a particular trait or disorder." For example, researchers may look at several identical twins separated at birth with hereditary neuroticism and determine the concordance rate of those diagnosed with generalized anxiety disorder. The percentage of twins diagnosed with generalized anxiety would be explained in the concordance percentage.

When analyzing the relationship between two variables influencing behavior, statisticians use the correlation coefficient . A correlation is "a statistical measurement of the relationship between two variables" and allows researchers to compare and contrast different variables that may influence behavior. A correlation coefficient is identified with a number scale from a negative one to a positive one that depicts the strength of the relationship between these two variables. Results are plotted on a graph to show their relationship visually. This measurement results in either a positive, negative, or zero correlation. The farther away a variable is plotted to zero, the stronger the relationship. For example, say researchers find that the symptoms of depression decrease when individuals conduct five hours of daily mediation. As the variable of meditation increases, the symptoms of depression decrease. This is a negative correlation because the two variables travel in different directions. If the daily meditation increases and the signs of depression increase, this would positively correlate as both variables increase in the same direction. If no change is found, this would be a zero correlation as there is no relationship between the two variables. Researchers use the correlation coefficient to compare and contrast behavioral traits between twins and the relationship between genetic and environmental variables.

Throughout human development, genetic makeup combined with lived experiences within one's environment influence the current behavior of the human race. Behavioral genetics helps researchers understand drivers of human behavior by examining the degree of influence these existing genetic traits and lived experiences within the environment have on behaviors. Research studies focus on the specifics of this behavior by concentrating on families, twins, and adoption . There is reason and value behind this research as it helps uncover the cause of human behavior, determining if genetics or environmental variables are the primary cause for specific behavior. Discovering the root cause of behavior also aids medical providers in focusing behavioral treatment on the root cause of the behavior problem.

The Minnesota Twin study is an excellent example of this research. It looked to determine when both identical or fraternal twins raised in the same households are, their behavior is influenced more by genetics or their environment. In these studies, researchers look closely at the genetic and environmental variables to determine if any relationship influences behavior. They look for a correlation coefficient to measure how a particular variable influences a specific trait in one twin compared to the other. Researchers describe the similarities and portions of identical twins suffering from mental illnesses such as depression using the statistical term concordance rate . In this research, family history is examined to look at past similar traits when determining mental illness hereditary traits. In some cases, when there is no previous history of a mental illness and an individual is the starting point of a genetic study, this person is referred to as the proband .

Video Transcript

Compared to your parents, how frequently do you tend to worry, and to experience negative emotions? What evidence is there for personality being at least partially determined by genetics? Consider this example: in the early 1950s, a German-British psychologist named Hans Jurgen Eysenck was the lead author on a published study about whether neuroticism is inherited. Neuroticism is a term that psychologists use to label a general tendency to experience negative emotions . Eysenck's test was a twin study , which used 25 pairs of fraternal twins , and 25 pairs of identical twins . Identical twins have the same genes, whereas fraternal twins have only about half the same genes. So, here's the value of a twin study like this: if identical twins are raised in the same households, and fraternal twins are raised in the same households, then characteristics that are determined largely by environmental factors should be about equally pronounced in both kinds of twins. By contrast, assuming again that both identical twins and fraternal twins are each raised in the same households, characteristics that are more influenced by genetics should be more similar between identical twins than between fraternal ones. In other words, the goal of identical vs. fraternal twin studies is to understand the significance of genetics for a particular characteristic. Identical twins share twice as much genetic code as fraternal twins, so by comparing the degree of similarity between the two different kinds of twins, researchers can better understand the role that genetics play in determining that characteristic. Eysenck's study found that neuroticism seemed to be overwhelmingly determined by genetics, with about 80% stemming from biological factors, and only 20% from environmental ones. This is normal: twin studies like this usually find more similarity between identical than between fraternal twins.

Twin studies such as Eyesenck's belong within a field known as behavioral genetics , which studies the heritability of traits in animals and humans. Behavioral genetics is not exclusively a sub-field of psychology; it's more of an interdisciplinary field that impinges on disciplines including psychology, biology and statistics. And studies in behavioral genetics do rely heavily on statistics. For example, twin studies often must establish concordance rates , which is the proportion of twin pairs in which both individuals exhibit the trait being considered. Now, with some traits, such as red hair, you either have it or you don't. With others, though, such as height, you can exhibit the trait to varying degrees. With these traits that can differ in degree, another type of statistic is needed. It's called a correlation coefficient . You don't need to worry about how to calculate it at the moment; just know that a correlation coefficient measures the extent to which the trait in one twin compares or contrasts with the same trait in the other twin.

Aside from twin studies, another type of research method that behavioral geneticists use to study humans is a family study . Probably you can guess from the name what this type of study is. A family study begins by isolating a single trait, or even a single illness, in a single person. This first person is known as the proband . Then that person's parents, siblings and children are studied with reference to the same trait. These relatives are known as first-degree relatives . Sometimes, family studies will even consider more distant relatives, such as aunts and grandparents, and so forth. These more distant relatives are known as second-degree relatives . Obviously, family studies aim to identify whether a trait runs in a family. But can you think of a complication with them? Families tend not only to be similar in terms of genetic makeup; they also tend to live in similar, if not the same, environments. So even though family studies can determine whether a given trait runs in a given family, they cannot easily determine whether it runs in the family due to environment or due to genetics. Twin studies, such as Eyesenck's, are better for distinguishing that.

Another variation of a family study is an adoption study . Adoption studies can happen in two different ways. First, biologically related people who have been raised in different environments can be studied. For example, what if identical twins were adopted by different families, and raised in different households? Since they have the same DNA, studies like these are well-positioned for drawing conclusions about effects those different environments have exerted on those twins. One well-known example of a study like this is known as the Minnesota Twin Study . It studied a pair of identical twins who were raised separately, and who did not meet until they were 40. The results were astonishing. One researcher concluded, '(On) multiple measures of personality and temperament, occupational and leisure-time interests and social attitudes, (identical) twins reared apart are about as similar as (identical) twins reared together.' Second, adopted children can be compared to their adopted parents, to their biological parents, or both. The Texas Adoption Project is one well-known adoption study.

To summarize, behavioral genetics is a field that attempts to ascertain whether, or the extent to which, traits, including personality traits, are inherited. To help determine this, researchers study families in family studies, twin studies and adoption studies. These studies are heavily reliant on statistical measurements, including concordance rates , which indicate the proportions of pairs in given studies in which both individuals exhibit the trait in question, and correlation coefficients. A correlation coefficient measures the extent to which the prevalence of a trait in one person matches or diverges from the prevalence of the same trait in another.

Lesson Objectives

After watching this lesson, you should be able to:

• Explain the goal of behavioral genetics
• Describe twin studies, family studies and adoption studies
• Define the terms concordance rate and correlation coefficient and understand how they are used in behavioral genetics

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Module 3: Prenatal Development

Behavioral genetics, learning outcomes.

• Describe the interaction between genetics and the environment
• Compare monozygotic and dizygotic twins

Behavioral geneticists study how individual differences arise, in the present, through the interaction of genes and the environment. When studying human behavior, behavioral geneticists often employ twin and adoption studies to research questions of interest. Twin studies compare the rates that a given behavioral trait is shared among identical and fraternal twins; adoption studies compare those rates among biologically related relatives and adopted relatives. Both approaches provide some insight into the relative importance of genes and environment for the expression of a given trait.

Nature or Nurture?

For decades, scholars have carried on the “nature/nurture” debate. For any particular feature, those on the “nature” side would argue that heredity plays the most important role in bringing about that feature. Those on the “nurture” side would argue that one’s environment is most significant in shaping the way we are. This debate continues in questions about what makes us masculine or feminine (Lippa, 2002), concerns about vision (Mutti, Kadnik, & Adams, 1996), and many other developmental issues.

Most scholars agree that there is a constant interplay between the two forces. It is difficult to isolate the root of any single behavior as a result solely of nature or nurture, and most scholars believe that even determining the extent to which nature or nurture impacts a human feature is difficult to answer. In fact, almost all human features are polygenic (a result of many genes) and multifactorial (a result of many factors, both genetic and environmental). It is as if one’s genetic make-up sets up a range of possibilities, which may or may not be realized depending upon one’s environmental experiences. For instance, a person might be genetically predisposed to develop diabetes, but the person’s lifestyle may help bring about the disease.

When you think about your own family history, it is easy to see that there are certain personality traits, behavioral characteristics, and medical conditions that are more common than others. This is the reason that doctors ask you about your family medical history. While genetic predisposition is important to consider, there are some family members who, for a variety of reasons, seemed to defy the odds of developing these conditions. These differences can be explained in part by the effect of epigenetic (above the genome) changes.

This video explains some of the research that gives insights into the complicated relationship between nature and nurture.

You can view the transcript for “Epigenetics: Nature vs nurture” here (opens in new window) .

The Epigenetic Framework

The term “epigenetic” has been used in developmental psychology to describe psychological development as the result of an ongoing, bi-directional interchange between heredity and the environment. Gottlieb (1998; 2000; 2002) suggests an analytic framework for the nature/nurture debate that recognizes the interplay between the environment, behavior, and genetic expression. This bidirectional interplay suggests that the environment can affect the expression of genes just as genetic predispositions can impact a person’s potentials. Likewise, environmental circumstances can trigger symptoms of a genetic disorder. For example, a person predisposed genetically for type 2 diabetes may trigger the disease through poor diet and little exercise.

The developmental psychologist Erik Erikson wrote of an  epigenetic principle  in his book  Identity: Youth and Crisis  (1968), encompassing the notion that we develop through an unfolding of our personality in predetermined stages, and that our environment and surrounding culture influence how we progress through these stages. This biological unfolding in relation to our socio-cultural settings is done in stages of psychosocial development, where “progress through each stage is in part determined by our success, or lack of success, in all the previous stages.”

In typical human families, children’s biological parents raise them, so it is very difficult to know whether children act like their parents due to genetic (nature) or environmental (nurture) reasons. Nevertheless, despite our restrictions on setting up human-based experiments, we do see real-world examples of nature-nurture at work in the human sphere—though they only provide partial answers to our many questions. The science of how genes and environments work together to influence behavior is called  behavioral genetics . The easiest opportunity we have to observe this is the  adoption study . When children are put up for adoption, the parents who give birth to them are no longer the parents who raise them. Children aren’t assigned to random adoptive parents in order to suit the particular interests of a scientist but adoption still tells us some interesting things, or at least confirms some basic expectations. For instance, if the biological child of tall parents were adopted into a family of short people, do you suppose the child’s growth would be affected? What about the biological child of a Spanish-speaking family adopted at birth into an English-speaking family? What language would you expect the child to speak? And what might these outcomes tell you about the difference between height and language in terms of nature-nurture?

Monozygotic and Dizygotic Twins

Another option for observing nature-nurture in humans involves  twin studies . To analyze nature–nurture using twins, we compare the similarity of monozygotic and dizygotic pairs. Monozygotic twins occur when a single zygote or fertilized egg splits apart in the first two weeks of development. The result is the creation of two separate but genetically identical offspring. About one-third of twins are monozygotic twins. Monozygotic twins occur in birthing at a rate of about 3 in every 1000 deliveries worldwide (about 0.3% of the world population). Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development. The children of monozygotic twins test genetically as half-siblings (or full siblings, if a pair of monozygotic twins reproduces with another pair of identical twins or with the same person), rather than first cousins.

Sometimes two eggs or ova are released and fertilized by two separate sperm. The result is  dizygotic  or fraternal twins. About two-thirds of twins are dizygotic. These two individuals share the same amount of genetic material as would any two children from the same mother and father. Older mothers are more likely to have dizygotic twins than are younger mothers and couples who use fertility drugs are also more likely to give birth to dizygotic twins. Consequently, there has been an increase in the number of fraternal twins in recent years (Bortolus et al., 1999). In vitro fertilization (IVF) techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000.

In the uterus, a majority of monozygotic twins (60–70%) share the same placenta but have separate amniotic sacs. The  placenta  is a temporary organ that connects the developing fetus via the umbilical cord to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange via the mother’s blood supply.  The  amniotic sac (also called the bag of waters or the membranes),  is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shortly before birth. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac. Fraternal twins each have their own placenta and own amniotic sac.

Figure 1.  Monozygotic twins come from a single zygote and generally share the same placenta, although some (18-30%) have separate placentas. Dizygotic twins come from two separately fertilized eggs and have their own placentas and amniotic sacs.

Monozygotic (one egg/identical) twins can be categorized into four types depending on the timing of the separation and duplication of cells. Various types of chorionicity and amniosity (how the baby’s sac looks) in monozygotic twins are a result of when the fertilized egg divides. This is known as placentation.

Figure 2. Various types of chorionicity and amniosity (how the baby’s sac looks) in monozygotic (one egg/identical) twins as a result of when the fertilized egg divides (Author Kevin Dufenbach)

Conjoined twins

Conjoined twins are monozygotic twins whose bodies are joined together during pregnancy. This occurs when the zygote starts to split after day 12  following fertilization and fails to separate completely. This condition occurs in about 1 in 50,000 human pregnancies. Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as the brain, heart or liver.

Vanishing twins

Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term because the other fetus has died very early in the pregnancy and has not been detected or recorded.  Early obstetric ultrasonography exams sometimes reveal an “extra” fetus, which fails to develop and instead disintegrates and vanishes in the uterus. There are several reasons for the “vanishing” fetus, including it being embodied or absorbed by the other fetus, placenta or the mother. This is known as vanishing twin syndrome. Also, in an unknown proportion of cases, two zygotes may fuse soon after fertilization, resulting in a single chimeric embryo, and, later, fetus.

Twin Studies

Using the features of height and spoken language as examples, let’s take a look at how nature and nurture apply: identical twins, unsurprisingly, are almost perfectly similar for height. The heights of fraternal twins, however, are like any other sibling pairs: more similar to each other than to people from other families, but hardly identical. This contrast between twin types gives us a clue about the role genetics plays in determining height.

Figure 3.  Identical twins Laurent and Larry Nicolas Bourgeois, also known as the Les Twins, are internationally renowned dancers.

Now consider spoken language. If one identical twin speaks Spanish at home, the co-twin with whom she is raised almost certainly does too. But the same would be true for a pair of fraternal twins raised together. In terms of spoken language, fraternal twins are just as similar as identical twins, so it appears that the genetic match of identical twins doesn’t make much difference.

Twin and adoption studies are two instances of a much broader class of methods for observing nature-nurture called  quantitative genetics , the scientific discipline in which similarities among individuals are analyzed based on how biologically related they are. We can do these studies with siblings and half-siblings, cousins, and twins who have been separated at birth and raised separately (Bouchard, Lykken, McGue, & Segal, 1990). Such twins are very rare and play a smaller role than is commonly believed in the science of nature–nurture, or with entire extended families (Plomin, DeFries, Knopik, & Neiderhiser, 2012).

It would be satisfying to be able to say that nature–nurture studies have given us conclusive and complete evidence about where traits come from, with some traits clearly resulting from genetics and others almost entirely from environmental factors, such as child-rearing practices and personal will; but that is not the case. Instead,  everything  has turned out to have some footing in genetics. The more genetically-related people are, the more similar they are—for  everything : height, weight, intelligence, personality, mental illness, etc. Sure, it seems like common sense that some traits have a genetic bias. For example, adopted children resemble their biological parents even if they have never met them, and identical twins are more similar to each other than are fraternal twins. And while certain psychological traits, such as personality or mental illness (e.g., schizophrenia), seem reasonably influenced by genetics, it turns out that the same is true for political attitudes, how much television people watch (Plomin, Corley, DeFries, & Fulker, 1990), and whether or not they get divorced (McGue & Lykken, 1992).

• Modification, adaptation, and original content. Authored by : Julie Lazzara for Lumen Learning. Provided by : Lumen Learning. License : CC BY: Attribution
• Psyc 200 Lifespan Psychology. Authored by : Laura Overstreet. Located at : http://opencourselibrary.org/econ-201/ . License : Public Domain: No Known Copyright
• The Nature Nurture Question. Provided by : Lumen Learning. Located at : https://courses.lumenlearning.com/waymaker-psychology/chapter/the-nature-nurture-question/ . License : CC BY: Attribution
• Epigenetics. Provided by : Wikipedia. Located at : https://en.wikipedia.org/wiki/Epigenetics . License : CC BY-SA: Attribution-ShareAlike
• Les Twins. Authored by : Shawn Welling. Provided by : Wikipedia. Located at : https://en.wikipedia.org/wiki/File:Les_Twins_profile.jpg . License : CC BY-SA: Attribution-ShareAlike
• Twins. Provided by : Wikipedia. Located at : https://en.wikipedia.org/wiki/Twin . License : CC BY-SA: Attribution-ShareAlike
• Epigenetics: Nature vs nurture. Authored by : Det medisinske fakultet UiO. Located at : https://www.youtube.com/watch?v=k50yMwEOWGU . License : Other . License Terms : Standard YouTube License

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behaviour genetics

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• National Center for Biotechnology Information - PubMed Central - Behavioral Genetics and Child Temperament
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behaviour genetics , the study of the influence of an organism’s genetic composition on its behaviour and the interaction of heredity and environment insofar as they affect behaviour. The question of the determinants of behavioral abilities and disabilities has commonly been referred to as the “nature-nurture” controversy.

The relationship between behaviour and genetics , or heredity, dates to the work of English scientist Sir Francis Galton (1822–1911), who coined the phrase “nature and nurture.” Galton studied the families of outstanding men of his day and concluded, like his cousin Charles Darwin , that mental powers run in families. Galton became the first to use twins in genetic research and pioneered many of the statistical methods of analysis that are in use today. In 1918 British statistician and geneticist Ronald Aylmer Fisher published a paper that showed how Gregor Mendel ’s laws of inheritance applied to complex traits influenced by multiple genes and environmental factors.

The first human behavioral genetic research on intelligence and mental illness began in the 1920s, when environmentalism (the theory that behaviour is a result of nongenetic factors such as various childhood experiences) became popular and before Nazi Germany’s abuse of genetics made the notion of hereditary influence abhorrent . Although genetic research on human behaviour continued throughout the following decades, it was not until the 1970s that a balanced view came to prevail in psychiatry that recognized the importance of nature as well as nurture. In psychology , this reconciliation did not take hold until the 1980s. Much behavioral genetic research today focuses on identifying specific genes that affect behavioral dimensions, such as personality and intelligence , and disorders, such as autism, hyperactivity, depression , and schizophrenia.

Quantitative genetic methods are used to estimate the net effect of genetic and environmental factors on individual differences in any complex trait , including behavioral traits. In addition, molecular genetic methods are used to identify specific genes responsible for genetic influence. Research is carried out in both animals and humans; however, studies using animal models tend to provide more-accurate data than studies in humans because both genes and environment can be manipulated and controlled in the laboratory.

By mating related animals such as siblings for many generations, nearly pure strains are obtained in which all offspring are genetically highly similar. It is possible to screen for genetic influence on behaviour by comparing the behaviour of different inbred strains raised in the same laboratory environment. Another method, known as selective breeding , evaluates genetic involvement by attempting to breed for high and low extremes of a trait for several generations. Both methods have been applied to a wide variety of animal behaviours, especially learning and behavioral responses to drugs , and this research provides evidence for widespread influence of genes on behaviour.

Because genes and environments cannot be manipulated in the human species, two quasi-experimental methods are used to screen for genetic influence on individual differences in complex traits such as behaviour. The twin method relies on the accident of nature that results in identical (monozygotic, MZ) twins or fraternal (dizygotic, DZ) twins. MZ twins are like clones , genetically identical to each other because they came from the same fertilized egg. DZ twins, on the other hand, developed from two eggs that happened to be fertilized at the same time. Like other siblings, DZ twins are only half as similar genetically as MZ twins. To the extent that behavioral variability is caused by environmental factors, DZ twins should be as similar for the behavioral trait as are MZ twins because both types of twins are reared by the same parents in the same place at the same time. If the trait is influenced by genes, then DZ twins ought to be less similar than MZ twins. For schizophrenia , for example, the concordance (risk of one twin’s being schizophrenic if the other is) is about 45 percent for MZ twins and about 15 percent for DZ twins. For intelligence as assessed by IQ tests, the correlation, an index of resemblance (0.00 indicates no resemblance and 1.00 indicates perfect resemblance), is 0.85 for MZ twins and 0.60 for DZ twins for studies throughout the world of more than 10,000 pairs of twins. The twin method has been robustly defended as a rough screen for genetic influence on behaviour.

The adoption method is a quasi-experimental design that relies on a social accident in which children are adopted away from their biological (birth) parents early in life, thus cleaving the effects of nature and nurture. Because the twin and adoption methods are so different, greater confidence is warranted when results from these two methods converge on the same conclusion—as they usually do. An influential adoption study of schizophrenia in 1966 by American behavioral geneticist Leonard Heston showed that children adopted away from their schizophrenic biological mothers at birth were just as likely to become schizophrenic (about 10 percent) as were children reared by their schizophrenic biological mothers. A 20-year study begun in the 1970s in the United States of intelligence of adopted children and their biological and adoptive parents showed increasing similarity from infancy to childhood to adolescence between the adopted children and their biological parents but no resemblance between the adopted children and their adoptive parents.

In contrast to traditional molecular genetic research that focused on rare disorders caused by a single genetic mutation , molecular genetic research on complex behavioral traits and common behavioral disorders is much more difficult because multiple genes are involved and each gene has a relatively small effect. However, some genes identified in animal models have contributed to an improved understanding of complex human behavioral disorders such as reading disability, hyperactivity, autism , and dementia .

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4.3 Behavioral Genetics

Learning outcomes.

• Describe the interaction between genetics and the environment
• Compare monozygotic and dizygotic twins

Behavioral geneticists study how individual differences arise, in the present, through the interaction of genes and the environment. When studying human behavior, behavioral geneticists often employ twin and adoption studies to research questions of interest. Twin studies compare the rates that a given behavioral trait is shared among identical and fraternal twins; adoption studies compare those rates among biologically related relatives and adopted relatives. Both approaches provide some insight into the relative importance of genes and environment for the expression of a given trait.

Nature or Nurture?

For decades, scholars have carried on the “nature/nurture” debate. For any particular feature, those on the “nature” side would argue that heredity plays the most important role in bringing about that feature. Those on the “nurture” side would argue that one’s environment is most significant in shaping the way we are. This debate continues in questions about what makes us masculine or feminine (Lippa, 2002), concerns about vision (Mutti et al. 1996), and many other developmental issues.

Most scholars agree that there is a constant interplay between the two forces. It is difficult to isolate the root of any single behavior as a result solely of nature or nurture, and most scholars believe that even determining the extent to which nature or nurture impacts a human feature is difficult to answer. In fact, almost all human features are polygenic (a result of many genes) and multifactorial (a result of many factors, both genetic and environmental). It is as if one’s genetic make-up sets up a range of possibilities, which may or may not be realized depending upon one’s environmental experiences. For instance, a person might be genetically predisposed to develop diabetes, but the person’s lifestyle may help bring about the disease.

When you think about your own family history, it is easy to see that there are certain personality traits, behavioral characteristics, and medical conditions that are more common than others. This is the reason that doctors ask you about your family medical history. While genetic predisposition is important to consider, there are some family members who, for a variety of reasons, seemed to defy the odds of developing these conditions. These differences can be explained in part by the effect of epigenetic (above the genome) changes.

You can view the transcript for “Epigenetics: Nature vs nurture” here (opens in new window) .

The Epigenetic Framework

The term “epigenetic” has been used in developmental psychology to describe psychological development as the result of an ongoing, bi-directional interchange between heredity and the environment. Gottlieb (1998; 2000; 2002) suggests an analytic framework for the nature/nurture debate that recognizes the interplay between the environment, behavior, and genetic expression. This bidirectional interplay suggests that the environment can affect the expression of genes just as genetic predispositions can impact a person’s potentials. Likewise, environmental circumstances can trigger symptoms of a genetic disorder. For example, a person predisposed genetically for type 2 diabetes may trigger the disease through poor diet and little exercise.

The developmental psychologist Erik Erikson wrote of an  epigenetic principle  in his book  Identity: Youth and Crisis  (1968), encompassing the notion that we develop through an unfolding of our personality in predetermined stages, and that our environment and surrounding culture influence how we progress through these stages. This biological unfolding in relation to our socio-cultural settings is done in stages of psychosocial development, where “progress through each stage is in part determined by our success, or lack of success, in all the previous stages.”

In typical human families, children’s biological parents raise them, so it is very difficult to know whether children act like their parents due to genetic (nature) or environmental (nurture) reasons. Nevertheless, despite our restrictions on setting up human-based experiments, we do see real-world examples of nature-nurture at work in the human sphere—though they only provide partial answers to our many questions. The science of how genes and environments work together to influence behavior is called  behavioral genetics . The easiest opportunity we have to observe this is the  adoption study . When children are put up for adoption, the parents who give birth to them are no longer the parents who raise them. Children aren’t assigned to random adoptive parents in order to suit the particular interests of a scientist but adoption still tells us some interesting things, or at least confirms some basic expectations. For instance, if the biological child of tall parents were adopted into a family of short people, do you suppose the child’s growth would be affected? What about the biological child of a Spanish-speaking family adopted at birth into an English-speaking family? What language would you expect the child to speak? And what might these outcomes tell you about the difference between height and language in terms of nature-nurture?

Monozygotic and Dizygotic Twins

Another option for observing nature-nurture in humans involves  twin studies . To analyze nature–nurture using twins, we compare the similarity of monozygotic and dizygotic pairs. Monozygotic twins occur when a single zygote or fertilized egg splits apart in the first two weeks of development. The result is the creation of two separate but genetically identical offspring. About one-third of twins are monozygotic twins. Monozygotic twins occur in birthing at a rate of about 3 in every 1000 deliveries worldwide (about 0.3% of the world population). Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development. The children of monozygotic twins test genetically as half-siblings (or full siblings, if a pair of monozygotic twins reproduces with another pair of identical twins or with the same person), rather than first cousins.

Sometimes two eggs or ova are released and fertilized by two separate sperm. The result is  dizygotic or fraternal twins. About two-thirds of twins are dizygotic. These two individuals share the same amount of genetic material as would any two children from the same two parents. Older gestational parents are more likely to have dizygotic twins than are younger gestational parents and couples who use fertility drugs are also more likely to give birth to dizygotic twins. Consequently, there has been an increase in the number of fraternal twins in recent years (Bortolus et al., 1999). In vitro fertilization (IVF) techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000.

In the uterus, a majority of monozygotic twins (60–70%) share the same placenta but have separate amniotic sacs. The  placenta is a temporary organ that connects the developing fetus via the umbilical cord to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange via the gestational parent’s blood supply.  The amniotic sac (also called the bag of waters or the membranes),  is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shortly before birth. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac. Fraternal twins each have their own placenta and own amniotic sac.

Monozygotic (one egg/identical) twins can be categorized into four types depending on the timing of the separation and duplication of cells. Various types of chorionicity and amniosity (how the baby’s sac looks) in monozygotic twins are a result of when the fertilized egg divides. This is known as placentation.

Conjoined twins

Conjoined twins are monozygotic twins whose bodies are joined together during pregnancy. This occurs when the zygote starts to split after day 12  following fertilization and fails to separate completely. This condition occurs in about 1 in 50,000 human pregnancies. Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as the brain, heart or liver.

Vanishing twins

Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term because the other fetus has died very early in the pregnancy and has not been detected or recorded. Early obstetric ultrasonography exams sometimes reveal an “extra” fetus, which fails to develop and instead disintegrates and vanishes in the uterus. There are several reasons for the “vanishing” fetus, including it being embodied or absorbed by the other fetus, placenta, or the gestational parent. This is known as vanishing twin syndrome. Also, in an unknown proportion of cases, two zygotes may fuse soon after fertilization, resulting in a single chimeric embryo, and, later, fetus.

Twin Studies

Using the features of height and spoken language as examples, let’s take a look at how nature and nurture apply: identical twins, unsurprisingly, are almost perfectly similar for height. The heights of fraternal twins, however, are like any other sibling pairs: more similar to each other than to people from other families, but hardly identical. This contrast between twin types gives us a clue about the role genetics plays in determining height.

Now consider spoken language. If one identical twin speaks Spanish at home, the co-twin with whom she is raised almost certainly does too. But the same would be true for a pair of fraternal twins raised together. In terms of spoken language, fraternal twins are just as similar as identical twins, so it appears that the genetic match of identical twins doesn’t make much difference.

Twin and adoption studies are two instances of a much broader class of methods for observing nature-nurture called  quantitative genetics , the scientific discipline in which similarities among individuals are analyzed based on how biologically related they are. We can do these studies with siblings and half-siblings, cousins, and twins who have been separated at birth and raised separately (Bouchard et al., 1990). Such twins are very rare and play a smaller role than is commonly believed in the science of nature–nurture, or with entire extended families (Plomin et al., 2012).

It would be satisfying to be able to say that nature–nurture studies have given us conclusive and complete evidence about where traits come from, with some traits clearly resulting from genetics and others almost entirely from environmental factors, such as child-rearing practices and personal will; but that is not the case. Instead,  everything  has turned out to have some footing in genetics. The more genetically-related people are, the more similar they are—for  everything : height, weight, intelligence, personality, mental illness, etc. Sure, it seems like common sense that some traits have a genetic bias. For example, adopted children resemble their biological parents even if they have never met them, and identical twins are more similar to each other than are fraternal twins. And while certain psychological traits, such as personality or mental illness (e.g., schizophrenia), seem reasonably influenced by genetics, it turns out that the same is true for political attitudes, how much television people watch (Plomin et al. 1990), and whether or not they get divorced (McGue & Lykken, 1992).

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Book contents

• Handbook of Research Methods in Social and Personality Psychology
• Copyright page
• Contributors
• Introduction to the Second Edition
• Introduction to the First Edition
• Chapter one Scratch an Itch with a Brick
• Part one Design and Inference Considerations
• Part two Procedural Possibilities
• Chapter six Using Physiological Indexes in Social Psychological Research
• Chapter seven Research Methods in Social and Affective Neuroscience
• Chapter eight Behavior Genetic Research Methods
• Chapter nine Methods of Small Group Research
• Chapter ten Inducing and Measuring Emotion and Affect
• Chapter eleven Complex Dynamical Systems in Social and Personality Psychology
• Chapter twelve Implicit Measures in Social and Personality Psychology
• Chapter thirteen The Mind in the Middle
• Chapter fourteen Behavioral Observation and Coding
• Chapter fifteen Methods for Studying Everyday Experience in Its Natural Context
• Chapter sixteen Survey Research
• Chapter seventeen Conducting Research on the Internet
• Part three Data Analytic Strategies
• Author Index
• Subject Index

Chapter eight - Behavior Genetic Research Methods

Testing Quasi-Causal Hypotheses Using Multivariate Twin Data

from Part two - Procedural Possibilities

Published online by Cambridge University Press:  05 June 2014

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• Behavior Genetic Research Methods
• By Eric Turkheimer , K. Paige Harden
• Edited by Harry T. Reis , University of Rochester, New York , Charles M. Judd , University of Colorado Boulder
• Book: Handbook of Research Methods in Social and Personality Psychology
• Online publication: 05 June 2014
• Chapter DOI: https://doi.org/10.1017/CBO9780511996481.012

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• Review Article
• Published: 11 May 2020

Using genetics for social science

• K. Paige Harden   ORCID: orcid.org/0000-0002-1557-6737 1 &
• Philipp D. Koellinger   ORCID: orcid.org/0000-0001-7413-0412 2  

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Social science genetics is concerned with understanding whether, how and why genetic differences between human beings are linked to differences in behaviours and socioeconomic outcomes. Our review discusses the goals, methods, challenges and implications of this research endeavour. We survey how the recent developments in genetics are beginning to provide social scientists with a powerful new toolbox they can use to better understand environmental effects, and we illustrate this with several substantive examples. Furthermore, we examine how medical research can benefit from genetic insights into social-scientific outcomes and vice versa. Finally, we discuss the ethical challenges of this work and clarify several common misunderstandings and misinterpretations of genetic research on individual differences.

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Acknowledgements

We thank C. Burik for preparing Fig. 1 and the Social Science Genetic Association Consortium ( https://www.thessgac.org/ ) for Fig. 3 . P.D.K. was financially supported by an ERC consolidator grant (647648 EdGe). K.P.H. was supported by the Jacobs Foundation, the Templeton Foundation and Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) grants R01-HD083613 and 5-R24-HD042849 (to the Population Research Center at the University of Texas at Austin). The funders had no role in the conceptualization, preparation or decision to publish this work.

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Harden, K.P., Koellinger, P.D. Using genetics for social science. Nat Hum Behav 4 , 567–576 (2020). https://doi.org/10.1038/s41562-020-0862-5

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Cover Story

A second look at twin studies

As behavioral genetics enters a second century, the field's oldest research method remains both relevant and controversial.

By LEA WINERMAN

Monitor Staff

April 2004, Vol 35, No. 4

Print version: page 46

"Twins have a special claim upon our attention; it is, that their history affords means of distinguishing between the effects of tendencies received at birth, and those that were imposed by the special circumstances of their after lives."

-- Sir Francis Galton, 19th century behavioral genetics pioneer, Inquiries into Human Faculty and its Development, 1875

More than a century after Galton's observation, twin studies remain a favorite tool of behavioral geneticists. Researchers have used twin studies to try to disentangle the environmental and genetic backgrounds of a cornucopia of traits, from aggression to intelligence to schizophrenia to alcohol dependence.

But despite the popularity of twin studies, some psychologists have long questioned assumptions that underlie them--like the supposition that fraternal and identical twins share equal environments or that people choose mates with traits unlike their own. The equal environments assumption, for example, has been debated for at least 40 years. Many researchers have found evidence that the assumption is valid, but others remain skeptical (see Further Reading below).

Overall, twin studies assumptions remain controversial, says psychologist James Jaccard, PhD, a psychologist who studies statistical methods at the University at Albany of the State University of New York. In response, though, researchers are working to expand and develop twin study designs and statistical methods. And while the assumptions question remains a stumbling block for some researchers, many agree twin studies will continue to be an important tool--along with emerging genome and molecular research methods (see article page 42 )--in shedding light on human behavioral genetics.

Methods and theory

The classical twin study design relies on studying twins raised in the same family environments. Monozygotic (identical) twins share all of their genes, while dizygotic (fraternal) twins share only about 50 percent of them. So, if a researcher compares the similarity between sets of identical twins to the similarity between sets of fraternal twins for a particular trait, then any excess likeness between the identical twins should be due to genes rather than environment.

Researchers use this method, and variations on it, to estimate the heritability of traits: The percentage of variance in a population due to genes. Modern twin studies also try to quantify the effect of a person's shared environment (family) and unique environment (the individual events that shape a life) on a trait.

The assumptions those studies rest on--questioned by some psychologists, including, in recent work, Jaccard--include:

Random mating. Twin researchers assume that people are as likely to choose partners who are different from themselves as they are to choose partners who are similar for a particular trait. If, instead, people tend to choose mates like themselves, then fraternal twins could share more than 50 percent of their genes--and hence more similarities on genetically influenced traits--because they would receive similar genes from their mothers and fathers.

Equal environments. Twin researchers also assume that fraternal and identical twins raised in the same homes experience equally similar environments. But some research suggests that parents, teachers, peers and others may treat identical twins more similarly than fraternal twins.

Gene-environment interaction. Some researchers think that interactions between genes and environment, rather than genes and environment separately, may influence many traits. A recent study from Science (Vol. 297, No. 5582) by Avshalom Caspi, PhD, of King's College London, for example, suggests that a gene might moderate propensity for violence, particularly in people who are severely maltreated as children. Many twin study designs don't take this type of complication into account.

Genetic mechanisms. Traits can be inherited through different genetic mechanisms. For traits governed by dominant genetic mechanisms, a dominant gene inherited from one parent trumps a recessive gene inherited from the other parent: If a person inherits a recessive gene for blue eyes from one parent and a dominant gene for brown eyes from the other parent, then the dominant brown gene wins, and the person's eyes are brown.

Additive genetic mechanisms, in contrast, mix together--a plant that receives one red gene and one white gene might, if the genes are additive, turn out pink. Epistatic mechanisms are complex cases where interactions among multiple genes may determine the outcome of one trait. Twin studies, in general, assume that only one type of genetic mechanism--usually additive--is operating for a particular trait.

The value of twin studies

Twin researchers acknowledge that these and other limitations exist. But, they say, the limitations don't negate the usefulness of twin studies. For traits that are substantially influenced by heredity, the approximately two-fold difference in genetic similarity between the two types of twins should outweigh any complications, says John Hewitt, PhD, director of the Institute for Behavioral Genetics at the University of Colorado at Boulder.

And the extent to which different assumptions matter may depend on which trait is being studied. Studies have suggested, for example, that people are more likely to select mates with similar levels of intelligence than they are mates with similar levels of neuroticism, extraversion and other personality traits (see page 50 ). So, researchers who use twins to study intelligence might have to worry more about nonrandom mating than researchers who study personality.

Twin study designs and statistical analysis methods are also constantly evolving and improving. The original twin study design has expanded to include studies of twins' extended families, longitudinal studies and other variations. Some of these variations allow researchers to address previous limitations--they can investigate the effects of nonrandom mating, for example, by including the spouses of twins in studies. In fact, says psychologist Dorret Boomsma, PhD, of Vrije Universiteit in the Netherlands, all of these assumptions can be tested, given the proper data. She argues that they should not be seen as assumptions at all, but instead as mechanisms whose relevance can be tested using study designs that go beyond the classical twin study design.

Analysis methods, likewise, don't remain static. "People are always thinking about ways to improve the analyses," Hewitt says.

Jaccard acknowledges that this is true. "For some designs, we don't have to make as strong assumptions as we used to make," he says. "Instead of having to assume away four constructs, we only have to assume away two or three."

In the age of molecular genetics, meanwhile, the classical twin study design is only one aspect of genetics research. Twin studies estimate the heritability of a trait, but molecular genetics attempts to pinpoint the effects of a particular gene.

The future of twin research will involve combining traditional twin studies with molecular genetics research, according to Hewitt, who believes that day is already here.

"When we conduct a study of twins these days, we always get DNA on everyone," Hewitt says. "And we'll use that DNA to try and identify specific individual genes that contribute to the overall pattern of heritability."

Further Reading

Boomsma, D., Busjahn, A., & Peltonen, L. (2002). Classical twin studies and beyond. Nature Reviews Genetics, 3 (11), 872-882.

Kendler, K.S., Neale, M.C., Kessler, R.C., Heath, A.C., & Eaves, L.J. (1993). A test of the equal-environment assumption in twin studies of psychiatric illness. Behavior Genetics, 23 , 21-28.

Neale, M.C., & Cardon, L.R. (1992). Methodology for genetic studies of twins and families. Dordrecht, The Netherlands: Kluwer Academic Press.

Pam, A., Kemker, S.S., Ross, C.A., & Golden, R. (1996). The "equal environments assumption" in MZ-DZ twin comparisons: An untenable premise of psychiatric genetics? Acta Geneticae Medicae et Gemellologiae, 45 (3), 349-360.

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Behavioral genetic methods

• PMID: 3712205
• DOI: 10.1111/j.1467-6494.1986.tb00394.x

Traditional behavioral genetic methods involve the use primarily of family, twin, and adoption correlations to estimate the relative contributions of genetic and environmental influences in the etiology of individual differences. These methods and representative results for personality are described. However, newer methods are emphasized: structural models and model-fitting, multivariate analysis, genetic change and continuity in development, shared and non-shared components of environmental variance, and genetic components of "environmental" variation. Because most applications of these behavioral genetic methods to the study of personality involve self-report omnibus questionnaires, an important direction for future research in this area is to use these methods to explore new issues and new measures that have emerged from personality theory and research during the past decade.

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THE BIOLOGY OF RELATIONSHIPS: WHAT BEHAVIORAL GENETICS TELLS US ABOUT INTERACTIONS AMONG FAMILY MEMBERS

Laura a. baker.

* Department of Psychology, University of Southern California

Introduction

Human behavior is subject to genetic variations. The ways in which individuals differ in their intellectual abilities, personalities, and mental health are, to a large extent, functions of their inherited genetic predispositions. Decades of research on twins, adoptees, and families have led to the inescapable conclusion that most reliably measured psychological characteristics are influenced to some degree by genes. Behavior also shows signs of genetic influence; the way one experiences stressful life events, for example, shows some genetic influence. Even personal aspects of individuals, such as spirituality and political ideology, are affected to an extent by genes. 1 It should come as no surprise, then, that genes influence the ways in which families function and how family members relate to one another. 2 Familial relationships of all kinds—parent-child, sibling, and spousal—can be shown to be at least partially the product of genetic factors.

This Article discusses a behavioral genetic perspective that provides insight into the biological factors that influence family relationships. Part II presents a brief overview of the research methods used to understand both genetic and environmental influences on human behavior. Part III then discusses several key findings from the field of behavioral sciences, particularly how they pertain to the ways in which family members relate to one another. It focuses on the following: (1) characteristics of parents and variations in the ways they treat their own children; (2) characteristics of children and how they may react to their parents’ behavior; and (3) the interactive processes that occur between parents and children. While the primary focus of Part III is on parent-child relationships, Part IV considers sibling and spousal relationships. Part V discusses the general interpretation of family relationships from a behavioral genetic perspective.

II. Twin, Family, and Adoption Studies

What is the evidence for the overwhelming influence of genetic factors on human psychological function? What does it mean to say that psychological dimensions of family relations are a function of genes? Answering these questions requires a basic understanding of behavioral genetic studies, which help to separate the effects of genes and environment in human behavior.

The general strategy in behavioral-genetic research designs involves the study of family members with varying degrees of genetic and environmental relatedness. 3 For example, genetic influences in a trait are evident if pairs of monozygotic (MZ) twins (who are genetically identical) are more similar to one another than dizygotic (DZ) twins (who share only about 50% of their genes), or if pairs of biological siblings raised together resemble one another more than unrelated (e.g., adoptive) siblings raised together. In general, if psychological traits and observed behavior have a genetic component, then genetically similar relatives should resemble one another more closely than individuals who share fewer genes. 4

Regarding environmental influences, researchers in behavioral genetics typically distinguish between two broad classes of effects: (1) environmental factors shared by relatives that cause them to behave similarly; and (2) unique, individual environments that are not shared by relatives, which cause them to be different from one another. These are referred to, respectively, as shared and nonshared environments. Shared environmental influences are evident when greater trait similarity is observed for those relatives who share more experiences (e.g., siblings raised together rather than apart), or when twins are more similar to one another than their genetic relatedness would predict. Evidence for nonshared environment often stems from differences observed between genetic relatives—that is, their lack of resemblance. Differences between MZ co-twins, for example, must stem from nonshared environments. The study of the similarities and differences between relatives of varying degrees of genetic and environmental relatedness provide the basic data for understanding the effects of genes, and thus the influence of shared and nonshared environments on behavior. 5

Within a few decades of the earliest twin, family, and adoption studies (which grew immensely from the 1970s onward), genetic factors were implicated in a wide range of human behaviors, such as cognitive ability and personality, as well as most major psychological disorders, such as depression and schizophrenia. 6 Collectively, these studies show that family members who are more closely related genetically demonstrate greater similarity than unrelated individuals for measured aspects of personality (e.g., extraversion or neuroticism), intellectual function (e.g., verbal skills and spatial ability), and likelihood of being diagnosed with a psychological disorder (e.g., depression or schizophrenia). 7

For a while, it was considered a challenge to find an enduring aspect of behavior that did not appear to be influenced by genes. Constructs such as religious behaviors and political attitudes, which had traditionally been understood to be strictly the product of culture, became the subject of behavioral genetic studies. Somewhat surprisingly, even these culturally defined behaviors appear to be influenced by genetic variations, at least within groups of individuals. For example, although one’s religion may be culturally defined and thus independent of genetic influences, the degree to which one engages in the rituals or adheres to the tenets of a particular religion appear to be affected by one’s genetic inheritance. 8 Indeed, even the degree to which an individual may endorse highly liberal or conservative ideals (e.g., abortion rights or gay rights) has been shown to be influenced by genetic factors; MZ twins are much more similar than DZ twins, and biological siblings are more similar than adoptive siblings in conservative attitudes from adolescence onward. 9

Around the same time these culturally defined behaviors became the subject of behavioral genetic research, investigators began to study other variables that were traditionally viewed as entirely “environmental” factors. This research challenged a long-standing social learning perspective in developmental psychology. What were traditionally considered to be “environmental” measures—including aspects of parenting—came to be understood as products of both genes and environment. Thus, we turn back now to the issue at hand: the various aspects of family relationships and how they are influenced by the complex interplay between genes and environment.

III. How the Behavior of Parents and children Is Influenced by Genes and Environment

Genes influence each individual’s behavioral and psychological characteristics, including intellectual ability, personality, and risk for mental illness—all of which have bearing on both parents and children within a family. The ways in which genes and environment can affect parent-child relationships can be seen in Figure 1 . This model represents a standard way in which behavioral geneticists think about human behavior in the context of family relationships. Parents’ genes influence their own behavior (including the ways they parent their children) and children’s genes influence their own behavior (including the ways they respond to their parents). The transmission of genes from parent to child is one important link that will lead to similarities between the behavior of a parent and a child. For example, to the extent that genes predispose an individual toward aggressive behavior, including violence toward others, parents and children will show similarities in this area of behavior. This might offer another explanation for the “cycle of violence” in which abusive parenting is related to aggression and other antisocial behaviors in children. 10 Antisocial behavior does, in fact, show moderate genetic influence in a wide range of studies. 11

Behavioral Genetic Model of Parent-Child Behavior

Besides direct genetic transmission, the model in Figure 1 indicates two other important ways in which the behavior of parents and children may be linked. First, parental behavior may itself be an important aspect of the child’s environment, which may be considered a form of “cultural transmission.” For example, a mother’s intelligence, personality, and mental health may have an impact on the child’s environment; mothers with higher intelligence and education spend more time reading to their children and engaging them in stimulating activities. 12 Importantly, however, these characteristics may each be influenced by the mother’s genetic makeup, and thus it can be seen how the mother’s genotype may ultimately be associated with the child’s environment. The association between genes and environment is generally referred to as a genotype-environment correlation (r GE ). One way in which r GE may arise is through this passive form of cultural transmission, which is referred to as a passive r GE . 13

The third link between the behavior of parents and children is established through the “evocative responses” that children’s behavior may elicit from their parents. Because a child’s behavior is itself influenced by the child’s genes, genetically different children living in the same family may elicit different parenting responses. This may result in another form of a genotype-environment correlation, an r GE of an evocative form. 14 That is, genetically based differences among children (e.g., temperament characteristics) may evoke different responses from their parents (e.g., disciplinary styles). Thus, genes and environment may be intertwined in complex ways within parent-child relationships.

These complexities can be unraveled by twin, family, and adoption studies. Genetic influences on parenting behavior can be understood by examining the similarities and differences in adult twins’ parenting styles. The parenting styles of adult twins—as measured by positivity, negativity, and monitoring of their children—were more highly correlated for MZ than DZ twins. 15 Reviews of other studies show similar patterns, in which parents’ genes influence the ways in which they parent their children. 16

Evidence of parental behaviors evoked by children has been demonstrated by studying how parents respond differently to two or more children in the same family, such as twins and other siblings. DZ twin children, for example, have reported more differences than MZ twins in levels of affect and warmth received by their parents, 17 a finding that has been replicated by using reports from parents about their own behavior, as well as by observing parents interacting with their different children. 18 Studies of adopted children have also revealed evocative responses in the rearing parents as a function of the child’s genetic predispositions, as measured by characteristics in their birth parents. More coercive parenting and negative affect were reported by the adoptive parents of children born to more antisocial parents. 19 These genetically high risk children displayed more conduct problems as children and adolescents, 20 and thus may have elicited more negative parenting. The key point is that the direction of causality may not necessarily run from parent to child; when children elicit parental behaviors, it can move in the reverse direction.

Passive r GE effects are best understood in studies comparing parent-child relationships in adopted and nonadopted children. Since adopted children are not genetically related to their rearing parents, the passive r GE does not influence their similarity, because the parents’ genes are not linked to the children’s environments. If passive r GE effects arise, whether through cultural transmission effects or other mechanisms, 21 correlations between parenting characteristics and child outcomes should be stronger when parents are raising their own genetic children. In fact, one study of adoptive and nonadoptive families found that parents’ ratings of family cohesion, low conflict, and open communication about feelings in early childhood were associated with lower ratings of aggression at age seven, but only for nonadopted children. 22 This link between early environment and child outcome was not found for adopted children, suggesting that passive gene-environment correlations may exist in nonadoptive families that have increased similarity compared to adoptive families. 23

Like other areas of human behavior, parenting itself is subject to genetic influence. This means that “bad parenting” may itself be influenced by the parents’ genetic inheritance. Negative affect, over-control, and even abuse and neglect could be related to the genetic makeup of the parents. This does not mean that environmental factors are unimportant, nor does it make such behavior excusable. It just means that genes can explain parenting behavior to some degree.

It is almost certain that parenting has an environmental influence on children. The fact that parental behavior—including parenting style—may be influenced by genes does not imply that such behaviors have no environmental impact on the children that receive such parenting. What are the best methods for testing the true environmental mediation of the relationship between parent and child behavior? Behavioral genetic designs—adoption and extended twin studies—actually provide the ideal methods for identifying environmental effects while controlling for genetic factors. 24 Behavioral genetic studies have helped resolve the issue of genetic and environmental effects in abusive parenting and its relationship to later behavior problems in children by studying, for example, differences in the physical maltreatment of co-twins. Twin resemblance for maltreatment was substantial and equal for MZ pairs and DZ pairs, suggesting that children’s genetic differences did not elicit abusive parenting. This does not rule out the possibility, however, that parents’ genes may have influenced their abusive parenting. Most importantly, associations between abusive parenting and a child’s later antisocial behavior remained significant even after controlling for genetic differences in the children. 25 It is noteworthy that this genetically informative study provided convincing evidence of an environmental effect of abusive parenting on child outcomes.

The environmental effects of abuse on child development have also been shown to be exacerbated by a child’s genetic predispositions. Children who inherited a deleterious gene that causes a deficiency in monoamine oxidase (MAO-A) appear particularly vulnerable to physical maltreatment, compared to children with a normal MAO-A gene. 26 These findings underscore the importance of genotype X environment interactions, 27 in which genetic predispositions amplify environmental vulnerabilities and vice versa. We can expect that a more detailed understanding of this complex interplay between specific genetic mechanisms and measured environments will emerge over the next few years, as more studies begin to obtain DNA markers of genetic variations.

IV. Other Family Relationships

Behavioral geneticists have also studied family relationships beyond that of the parent and child. Sibling interactions, for example, have been examined in both twin and non-twin siblings. Unlike parents and children, who always share exactly half of their genes, siblings vary in their degree of genetic relatedness. MZ twins are genetically identical; DZ twins and non-twin siblings share about half of their genes, although some pairs may share more or less genetic material. This variation in genetic relatedness could explain why some siblings have a more cooperative and close relationship than others. Genetic similarity among siblings has been shown to affect both their positive and negative interactions with one another, 28 as well as levels of mutual competition and cooperation. 29 In general, siblings who share a stronger genetic makeup demonstrate a closer, more cooperative and positive relationship with one another.

Genetic variations among siblings living in the same family have also been suggested as an important source of differential parenting. The differential parenting of two siblings, albeit stemming originally from their genetic differences, has an environmental effect on the children’s psychological outcomes and may amplify sibling differences over time.

The quality of the relationship between marital partners has also been a subject of behavioral genetic studies. Twin similarity for marital satisfaction has been reported to be greater for MZ pairs than for DZ pairs, 30 suggesting the importance of individual genetic factors in determining the success of a marriage. Indeed, twin studies have also shown significantly greater concordance for divorce among MZ pairs than among DZ pairs, suggesting a substantial genetic effect on the likelihood of a failed marriage. 31 Genetically influenced personality traits, such as negative emotionality (i.e., neuroticism), are also predictive of divorce, and may explain much of the genetic risk for divorce. 32

V. Conclusion

One lesson to be learned from behavioral genetic studies of parenting and other types of family relationships is that one must be careful in drawing conclusions based on findings of family resemblance in nuclear, nonadoptive families. Consider the well-known finding that children of abusers are likely to become aggressive and violent, and perhaps even become abusive parents themselves later in life. 33 Although it is tempting to assume such resemblance is a function of learning and experience, it is possible that inherited genetic factors could explain the transmission of abuse across generations. Family resemblance for a given characteristic does not necessarily imply either genetic or environmental influence, since either could explain observed similarity among family members. Thus, the mere fact that children who are abused by their parents are more likely to become abusive themselves does not prove a causal relationship between parenting behaviors and child outcome. Through genetically controlled studies, we have come to understand that both genes and environment play a role in the cycle of violence. 34 Genes may predispose certain adults toward violence and aggression, even toward their own children. Such behaviors can in turn have a real environmental impact on the child’s mental health and on behavioral outcomes. Children’s genes may also predispose them toward oppositional and other antisocial behaviors, which may elicit negative parenting from the adults who are raising them.

The fact that genetic influences are crucially important for most areas of behavior does not mean that environmental influences are unimportant. Genes typically account for no more than one-half to two-thirds of the variation seen in most individual’s psychological traits. But most environmental influences are based on individual experiences and exposures that are not shared by family members. The implication for families is that most observed resemblance among its individual members is a function of their genetic similarity—not their shared experiences.

Finally, behavioral genetic studies of family relationships provide the valuable information required to develop effective programs of intervention and prevention of serious mental health and behavioral problems. Establishing that environmental effects unequivocally mediate links between parents’ and children’s behavior is a step toward ensuring the success of treatment programs that target either parents or children.

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Data and Statistics on Children’s Mental Health

Mental health is an important part of children’s overall health and well-being. Mental health includes children’s mental, emotional, and behavioral well-being. It affects how children think, feel, and act. It also plays a role in how children handle stress, relate to others, and make healthy choices.

Mental disorders among children are described as serious changes in the way children typically learn, behave, or handle their emotions, causing distress and problems getting through the day. 1 Among the more common mental disorders that can be diagnosed in childhood are attention-deficit/hyperactivity disorder (ADHD), anxiety, and behavior disorders.

There are different ways to assess mental health and mental disorders in children. CDC uses surveys, like the National Survey of Children’s Health, to describe the presence of positive indicators of children’s mental health and to understand the number of children with diagnosed mental disorders and whether they received treatment. In this type of survey, parents report on indicators of positive mental health for their child and report any diagnoses their child has received from a healthcare provider.  The information on this page provides data about indicators of positive mental health in children and mental health disorders that are most common in children.

Facts about mental health in U.S. children

National data on positive mental health indicators that describe mental, emotional, and behavioral well-being for children are limited. Based on the data we do have:

• Affection (97.0%), resilience (87.9%), positivity (98.7%) and curiosity (93.9%) among children ages 3-5 years 2
• Curiosity (93.0%), persistence (84.2%), and self-control (73.8%) among children ages 6-11 years 2
• Curiosity (86.5 %), persistence (84.7%), and self-control (79.8%) among children ages 12-17 years 2

Facts about mental disorders in U.S. children

• ADHD 9.8% (approximately 6.0 million) 2
• Anxiety 9.4% (approximately 5.8 million) 2
• Behavior problems 8.9% (approximately 5.5 million) 2
• Depression 4.4% (approximately 2.7 million) 2
• Having another mental disorder was most common in children with depression: about 3 in 4 children with depression also had anxiety (73.8%) and almost 1 in 2 had behavior problems (47.2%). 3
• For children with anxiety, more than 1 in 3 also had behavior problems (37.9%) and about 1 in 3 also had depression (32.3%). 3
• For children with behavior problems, more than 1 in 3 also had anxiety (36.6%) and about 1 in 5 also had depression (20.3%). 3
• “Ever having been diagnosed with either anxiety or depression” among children aged 6–17 years increased from 5.4% in 2003 to 8% in 2007 and to 8.4% in 2011–2012. 4
• “Ever having been diagnosed with anxiety” increased from 5.5% in 2007 to 6.4% in 2011–2012. 4
• “Ever having been diagnosed with depression” did not change between 2007 (4.7%) and 2011-2012 (4.9%). 4
• For adolescents, depression, substance use and suicide are important concerns. Among adolescents aged 12-17 years in 2018-2019 reporting on the past year:
• 15.1% had a major depressive episode. 2
• 36.7% had persistent feelings of sadness or hopelessness. 2
• 4.1% had a substance use disorder. 2
• 1.6% had an alcohol use disorder. 2
• 3.2% had an illicit drug use disorder. 2
• 18.8% seriously considered attempting suicide. 2
• 15.7% made a suicide plan. 2
• 8.9% attempted suicide. 2
• 2.5% made a suicide attempt requiring medical treatment. 2

Learn more about high-risk substance use among youth . Learn more about suicide .

• Nearly 8 in 10 children (78.1%) with depression received treatment. 3
• 6 in 10 children (59.3%) with anxiety received treatment. 3
• More than 5 in 10 children (53.5%) with behavior disorders received treatment. 3
• 1 in 6 U.S. children aged 2–8 years (17.4%) had a diagnosed mental, behavioral, or developmental disorder. 5
• Diagnoses of ADHD, anxiety, and depression become are more common with increased age. 3
• Behavior problems are more common among children aged 6–11 years than younger or older children. 3

• Among children aged 2-8 years, boys were more likely than girls to have a mental, behavioral, or developmental disorder. 5
• Among children living below 100% of the federal poverty level, more than 1 in 5 (22%) had a mental, behavioral, or developmental disorder. 5
• Age and poverty level affected the likelihood of children receiving treatment for anxiety, depression, or behavior problems. 3
• Children who were discriminated against based on race or ethnicity had higher percentages of one or more physical health conditions (37.8% versus 27.1%), and one or more mental health conditions (28.9% versus 17.8%). 6
• Racial/ethnic discrimination was almost seven times as common among children with three other ACEs compared to those with no other ACEs. 6

Note : The estimates reported on this page are based on parent report, using nationally representative surveys. This method has several limitations. It is not known to what extent children receive these diagnoses accurately. Estimates based on parent-reported diagnoses may match those based on medical records, 7  but some children may also have mental disorders that have not been diagnosed, or receive diagnoses that may not be the best fit for their symptoms. Limited information on measuring children’s mental health nationally is available 2 .

Read more about children’s mental health from a community study .

Access to mental health treatment

Early diagnosis and appropriate services for children and their families can make a difference in the lives of children with mental disorders. 7 Access to providers who can offer services, including screening, referrals, and treatment, varies by location. CDC is working to learn more about access to behavioral health services and supports for children and their families.

View information by state describing the rates of different types of providers who can offer behavioral health services providers by county.

Read a recent report describing shortages of services, barriers to treatment, and how integration of behavioral health care with pediatric primary care could address the issues.

Read a policy brief on potential ways to increase access to mental health services for children in rural areas

What is It and Why is It Important?

Data sources for mental health and related conditions

There are many different datasets which include information on children’s mental health and related conditions for children living in the United States.

Healthy People 2030 Healthy People 2030 sets data-driven national objectives to improve health and well-being over the next decade, including children’s mental health and well-being.

National Survey of Family Growth (NSFG) NSFG gathers information on family life, marriage and divorce, pregnancy, infertility, use of contraception, and general and reproductive health.

National Health and Nutrition Examination Survey (NHANES) NHANES assesses health and nutritional status through interviews and physical examinations, and includes conditions, symptoms, and concerns associated with mental health and substance abuse, as well as the use and need for mental health services.

National Health Interview Survey (NHIS) NHIS collects data on children’s mental health, mental disorders, such as ADHD, autism spectrum disorder, depression and anxiety problems, and use and need for mental health services.

National Survey of Children’s Health (NSCH) NSCH examines the health of children, with emphasis on well-being, including medical homes, family interactions, the health of parents, school and after-school experiences, and safe neighborhoods. This survey was redesigned in 2016.

For previous versions of this survey, see also: National Survey of Children’s Health (NSCH 2003, 2007, 2011-12) National Survey of Children with Special Healthcare Needs (NS-CSHCN 2001, 2005-6, 2009-10)

National Survey of the Diagnosis and Treatment of ADHD and Tourette Syndrome (NS-DATA) NS-DATA collects information about children, 2-15 years old in 2011-2012, who had ever been diagnosed with ADHD and/or Tourette syndrome (TS), with the goal of better understanding diagnostic practices, level of impairment, and treatments for this group of children.

National Survey on Drug Use and Health (NSDUH) NSDUH, administered by the Substance Abuse and Mental Health Services Administration (SAMHSA), provides national- and state-level data on the use of tobacco, alcohol, and illicit drugs (including non-medical use of prescription drugs), as well as data on mental health in the United States.

National Vital Statistics System (NVSS) NVSS contains vital statistics from the official records of live births, deaths, causes of death, marriages, divorces, and annulment recorded by states and independent registration areas

National Youth Tobacco Survey (NYTS) NYTS is a nationally representative school-based survey on tobacco use by public school students enrolled in grades 6-12.

School Associated Violent Death Study (SAVD) SAVD plays an important role in monitoring trends related to school-associated violent deaths (including suicide), identifying the factors that increase the risk, and assessing the effects of prevention efforts.

School Health Policies and Programs Study (SHPPS) SHPPS is a national survey assessing school health policies and practices at the state, district, school, and classroom levels. Collected data includes mental health and social service policies.

Web-based Injury Statistics Query and Reporting System (WISQARS) WISQARS is an interactive database system that provides customized reports of injury-related data.

Youth Risk Behavior Surveillance System (YRBSS) The YRBSS monitors health-risk behaviors, including tobacco use, substance abuse, unintentional injuries and violence, sexual behaviors that contribute to unintended pregnancy, and sexually transmitted diseases.

• Perou R, Bitsko RH, Blumberg SJ, Pastor P, Ghandour RM, Gfroerer JC, Hedden SL, Crosby AE, Visser SN, Schieve LA, Parks SE, Hall JE, Brody D, Simile CM, Thompson WW, Baio J, Avenevoli S, Kogan MD, Huang LN. Mental health surveillance among children – United States, 2005—2011. MMWR 2013;62(Suppl; May 16, 2013):1-35. [ Read summary ]
• Bitsko RH, Claussen AH, Lichtstein J, Black LJ, Everett Jones S, Danielson MD, Hoenig JM, Davis Jack SP, Brody DJ, Gyawali S, Maenner MM, Warner M, Holland KM, Perou R, Crosby AE, Blumberg SJ, Avenevoli S, Kaminski JW, Ghandour RM. Surveillance of Children’s Mental Health – United States, 2013 – 2019 MMWR, , 2022 / 71(Suppl-2);1–42. [Read article]
• Ghandour RM, Sherman LJ, Vladutiu CJ, Ali MM, Lynch SE, Bitsko RH, Blumberg SJ. Prevalence and treatment of depression, anxiety, and conduct problems in U.S. children. The Journal of Pediatrics , 2018. Published online before print  October 12, 2018 [ Read summary ]
• Bitsko RH, Holbrook JR, Ghandour RM, Blumberg SJ, Visser SN, Perou R, Walkup J. Epidemiology and impact of healthcare provider diagnosed anxiety and depression among US children. Journal of Developmental and Behavioral Pediatrics . Published online before print April 24, 2018 [ Read summary ]
• Cree RA, Bitsko RH, Robinson LR, Holbrook JR, Danielson ML, Smith DS, Kaminski JW, Kenney MK, Peacock G. Health care, family, and community factors associated with mental, behavioral, and developmental disorders and poverty among children aged 2–8 years — United States, 2016. MMWR , 2018;67(5):1377-1383. [ Read article ]
• Hutchins HJ, Barry CM, Valentine V, Bacon S, Njai R, Claussen AH, Ghandour RM, Lebrun-Harris LA, Perkins K, Robinson LR (submitted). Perceived racial/ethnic discrimination, physical and mental health conditions in childhood, and the relative role of other adverse experiences. Adversity and Resilience Science published online May 23, 2022. [ Read summary ]
• US Department of Health and Human Services Health Resources and Services Administration & Maternal and Child Health Bureau. Mental health: A report of the Surgeon General . Rockville, MD: US Department of Health and Human Services, Substance Abuse and Mental Health Services Administration, Center for Mental Health Services, and National Institutes of Health, National Institute of Mental Health; 1999. [ Read report ]

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• Patient Care & Health Information
• Diseases & Conditions
• Depression (major depressive disorder)
• What is depression? A Mayo Clinic expert explains.

Learn more about depression from Craig Sawchuk, Ph.D., L.P., clinical psychologist at Mayo Clinic.

Hi, I'm Dr. Craig Sawchuk, a clinical psychologist at Mayo Clinic. And I'm here to talk with you about depression. Whether you're looking for answers for yourself, a friend, or loved one, understanding the basics of depression can help you take the next step.

Depression is a mood disorder that causes feelings of sadness that won't go away. Unfortunately, there's a lot of stigma around depression. Depression isn't a weakness or a character flaw. It's not about being in a bad mood, and people who experience depression can't just snap out of it. Depression is a common, serious, and treatable condition. If you're experiencing depression, you're not alone. It honestly affects people of all ages and races and biological sexes, income levels and educational backgrounds. Approximately one in six people will experience a major depressive episode at some point in their lifetime, while up to 16 million adults each year suffer from clinical depression. There are many types of symptoms that make up depression. Emotionally, you may feel sad or down or irritable or even apathetic. Physically, the body really slows down. You feel tired. Your sleep is often disrupted. It's really hard to get yourself motivated. Your thinking also changes. It can just be hard to concentrate. Your thoughts tend to be much more negative. You can be really hard on yourself, feel hopeless and helpless about things. And even in some cases, have thoughts of not wanting to live. Behaviorally, you just want to pull back and withdraw from others, activities, and day-to-day responsibilities. These symptoms all work together to keep you trapped in a cycle of depression. Symptoms of depression are different for everyone. Some symptoms may be a sign of another disorder or medical condition. That's why it's important to get an accurate diagnosis.

While there's no single cause of depression, most experts believe there's a combination of biological, social, and psychological factors that contribute to depression risk. Biologically, we think about genetics or a family history of depression, health conditions such as diabetes, heart disease or thyroid disorders, and even hormonal changes that happen over the lifespan, such as pregnancy and menopause. Changes in brain chemistry, especially disruptions in neurotransmitters like serotonin, that play an important role in regulating many bodily functions, including mood, sleep, and appetite, are thought to play a particularly important role in depression. Socially stressful and traumatic life events, limited access to resources such as food, housing, and health care, and a lack of social support all contribute to depression risk. Psychologically, we think of how negative thoughts and problematic coping behaviors, such as avoidance and substance use, increase our vulnerability to depression.

The good news is that treatment helps. Effective treatments for depression exist and you do have options to see what works best for you. Lifestyle changes that improve sleep habits, exercise, and address underlying health conditions can be an important first step. Medications such as antidepressants can be helpful in alleviating depressive symptoms. Therapy, especially cognitive behavioral therapy, teaches skills to better manage negative thoughts and improve coping behaviors to help break you out of cycles of depression. Whatever the cause, remember that depression is not your fault and it can be treated.

To help diagnose depression, your health care provider may use a physical exam, lab tests, or a mental health evaluation. These results will help identify various treatment options that best fit your situation.

Help is available. You don't have to deal with depression by yourself. Take the next step and reach out. If you're hesitant to talk to a health care provider, talk to a friend or loved one about how to get help. Living with depression isn't easy and you're not alone in your struggles. Always remember that effective treatments and supports are available to help you start feeling better. Want to learn more about depression? Visit mayoclinic.org. Do take care.

Depression is a mood disorder that causes a persistent feeling of sadness and loss of interest. Also called major depressive disorder or clinical depression, it affects how you feel, think and behave and can lead to a variety of emotional and physical problems. You may have trouble doing normal day-to-day activities, and sometimes you may feel as if life isn't worth living.

More than just a bout of the blues, depression isn't a weakness and you can't simply "snap out" of it. Depression may require long-term treatment. But don't get discouraged. Most people with depression feel better with medication, psychotherapy or both.

Depression care at Mayo Clinic

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Although depression may occur only once during your life, people typically have multiple episodes. During these episodes, symptoms occur most of the day, nearly every day and may include:

• Feelings of sadness, tearfulness, emptiness or hopelessness
• Angry outbursts, irritability or frustration, even over small matters
• Loss of interest or pleasure in most or all normal activities, such as sex, hobbies or sports
• Sleep disturbances, including insomnia or sleeping too much
• Tiredness and lack of energy, so even small tasks take extra effort
• Reduced appetite and weight loss or increased cravings for food and weight gain
• Anxiety, agitation or restlessness
• Slowed thinking, speaking or body movements
• Feelings of worthlessness or guilt, fixating on past failures or self-blame
• Trouble thinking, concentrating, making decisions and remembering things
• Frequent or recurrent thoughts of death, suicidal thoughts, suicide attempts or suicide
• Unexplained physical problems, such as back pain or headaches

For many people with depression, symptoms usually are severe enough to cause noticeable problems in day-to-day activities, such as work, school, social activities or relationships with others. Some people may feel generally miserable or unhappy without really knowing why.

Depression symptoms in children and teens

Common signs and symptoms of depression in children and teenagers are similar to those of adults, but there can be some differences.

• In younger children, symptoms of depression may include sadness, irritability, clinginess, worry, aches and pains, refusing to go to school, or being underweight.
• In teens, symptoms may include sadness, irritability, feeling negative and worthless, anger, poor performance or poor attendance at school, feeling misunderstood and extremely sensitive, using recreational drugs or alcohol, eating or sleeping too much, self-harm, loss of interest in normal activities, and avoidance of social interaction.

Depression symptoms in older adults

Depression is not a normal part of growing older, and it should never be taken lightly. Unfortunately, depression often goes undiagnosed and untreated in older adults, and they may feel reluctant to seek help. Symptoms of depression may be different or less obvious in older adults, such as:

• Memory difficulties or personality changes
• Physical aches or pain
• Fatigue, loss of appetite, sleep problems or loss of interest in sex — not caused by a medical condition or medication
• Often wanting to stay at home, rather than going out to socialize or doing new things
• Suicidal thinking or feelings, especially in older men

When to see a doctor

If you feel depressed, make an appointment to see your doctor or mental health professional as soon as you can. If you're reluctant to seek treatment, talk to a friend or loved one, any health care professional, a faith leader, or someone else you trust.

When to get emergency help

If you think you may hurt yourself or attempt suicide, call 911 in the U.S. or your local emergency number immediately.

Also consider these options if you're having suicidal thoughts:

• Call your doctor or mental health professional.
• Contact a suicide hotline.
• In the U.S., call or text 988 to reach the 988 Suicide & Crisis Lifeline, available 24 hours a day, seven days a week. Or use the Lifeline Chat . Services are free and confidential.
• U.S. veterans or service members who are in crisis can call 988 and then press “1” for the Veterans Crisis Line . Or text 838255. Or chat online .
• The Suicide & Crisis Lifeline in the U.S. has a Spanish language phone line at 1-888-628-9454 (toll-free).
• Reach out to a close friend or loved one.
• Contact a minister, spiritual leader or someone else in your faith community.

If you have a loved one who is in danger of suicide or has made a suicide attempt, make sure someone stays with that person. Call 911 or your local emergency number immediately. Or, if you think you can do so safely, take the person to the nearest hospital emergency room.

More Information

Depression (major depressive disorder) care at Mayo Clinic

• Male depression: Understanding the issues
• Nervous breakdown: What does it mean?
• Pain and depression: Is there a link?

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It's not known exactly what causes depression. As with many mental disorders, a variety of factors may be involved, such as:

• Biological differences. People with depression appear to have physical changes in their brains. The significance of these changes is still uncertain, but may eventually help pinpoint causes.
• Brain chemistry. Neurotransmitters are naturally occurring brain chemicals that likely play a role in depression. Recent research indicates that changes in the function and effect of these neurotransmitters and how they interact with neurocircuits involved in maintaining mood stability may play a significant role in depression and its treatment.
• Hormones. Changes in the body's balance of hormones may be involved in causing or triggering depression. Hormone changes can result with pregnancy and during the weeks or months after delivery (postpartum) and from thyroid problems, menopause or a number of other conditions.
• Inherited traits. Depression is more common in people whose blood relatives also have this condition. Researchers are trying to find genes that may be involved in causing depression.
• Marijuana and depression
• Vitamin B-12 and depression

Risk factors

Depression often begins in the teens, 20s or 30s, but it can happen at any age. More women than men are diagnosed with depression, but this may be due in part because women are more likely to seek treatment.

Factors that seem to increase the risk of developing or triggering depression include:

• Certain personality traits, such as low self-esteem and being too dependent, self-critical or pessimistic
• Traumatic or stressful events, such as physical or sexual abuse, the death or loss of a loved one, a difficult relationship, or financial problems
• Blood relatives with a history of depression, bipolar disorder, alcoholism or suicide
• Being lesbian, gay, bisexual or transgender, or having variations in the development of genital organs that aren't clearly male or female (intersex) in an unsupportive situation
• History of other mental health disorders, such as anxiety disorder, eating disorders or post-traumatic stress disorder
• Abuse of alcohol or recreational drugs
• Serious or chronic illness, including cancer, stroke, chronic pain or heart disease
• Certain medications, such as some high blood pressure medications or sleeping pills (talk to your doctor before stopping any medication)

Complications

Depression is a serious disorder that can take a terrible toll on you and your family. Depression often gets worse if it isn't treated, resulting in emotional, behavioral and health problems that affect every area of your life.

Examples of complications associated with depression include:

• Excess weight or obesity, which can lead to heart disease and diabetes
• Pain or physical illness
• Alcohol or drug misuse
• Anxiety, panic disorder or social phobia
• Family conflicts, relationship difficulties, and work or school problems
• Social isolation
• Suicidal feelings, suicide attempts or suicide
• Self-mutilation, such as cutting
• Premature death from medical conditions
• Depression and anxiety: Can I have both?

There's no sure way to prevent depression. However, these strategies may help.

• Take steps to control stress, to increase your resilience and boost your self-esteem.
• Reach out to family and friends, especially in times of crisis, to help you weather rough spells.
• Get treatment at the earliest sign of a problem to help prevent depression from worsening.
• Consider getting long-term maintenance treatment to help prevent a relapse of symptoms.
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• Antidepressants: Selecting one that's right for you
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On the Usefulness of Behavior Genetics: Using Family Studies in Evolutionary Psychological Science to Improve Causal Inference and Sharpen Theory

• Published: 26 October 2023
• Volume 9 , pages 387–399, ( 2023 )

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• Brian B. Boutwell 1 &
• Dario Maestripieri 2  

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We argue that research in the psychological sciences testing evolutionarily informed questions could benefit considerably from more frequent use of techniques common in behavior genetics.

We review some of the reasons why data and analytical strategies in behavior genetics confer certain advantages over more traditional forms of data analysis. In particular, we focus on the wide availability of secondary data, the generalizability of data, the capacity of certain designs to bolster causal inference capabilities, and the overall adaptability of the research designs to a wide array of empirical questions.

Not only do we show how the use of sibling designs can be of methodological assistance, but we also demonstrate how they can play a role in refining theories informed by evolution. In order to give a more concrete vision of what this can look like, we offer a type of case study using prior work which has already taken advantage of behavior genetic tools.

Conclusions

Because of the efforts to situate psychological science in the context of evolutionary biology, the field has undergone considerable intellectual growth. We suggest that by simply making more frequent use of tools in behavior genetics, the fields of psychology might further accelerate the progress that is already well underway.

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Boutwell, B.B., Maestripieri, D. On the Usefulness of Behavior Genetics: Using Family Studies in Evolutionary Psychological Science to Improve Causal Inference and Sharpen Theory. Adaptive Human Behavior and Physiology 9 , 387–399 (2023). https://doi.org/10.1007/s40750-023-00228-9

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Published : 26 October 2023

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DOI : https://doi.org/10.1007/s40750-023-00228-9

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