{"id":63,"date":"2018-04-16T22:43:02","date_gmt":"2018-04-17T02:43:02","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/discoverpsychology2\/?post_type=chapter&p=63"},"modified":"2018-04-26T19:14:33","modified_gmt":"2018-04-26T23:14:33","slug":"chapter-13","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/discoverpsychology2\/chapter\/chapter-13\/","title":{"raw":"The Nature-Nurture Question","rendered":"The Nature-Nurture Question"},"content":{"raw":"
By Eric Turkheimer<\/a>\r\n

University of Virginia<\/p>\r\n\r\n<\/header>

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People have a deep intuition about what has been called the \u201cnature\u2013nurture question.\u201d Some aspects of our behavior feel as though they originate in our genetic makeup, while others feel like the result of our upbringing or our own hard work. The scientific field of behavior genetics attempts to study these differences empirically, either by examining similarities among family members with different degrees of genetic relatedness, or, more recently, by studying differences in the DNA of people with different behavioral traits. The scientific methods that have been developed are ingenious, but often inconclusive. Many of the difficulties encountered in the empirical science of behavior genetics turn out to be conceptual, and our intuitions about nature and nurture get more complicated the harder we think about them. In the end, it is an oversimplification to ask how \u201cgenetic\u201d some particular behavior is. Genes and environments always combine to produce behavior, and the real science is in the discovery of how they combine for a given behavior.<\/span><\/p>\r\n \r\n\r\n<\/section>

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Learning Objectives<\/h3>\r\n
    \r\n \t
  1. Understand what the nature\u2013nurture debate is and why the problem fascinates us.<\/li>\r\n \t
  2. Understand why nature\u2013nurture questions are difficult to study empirically.<\/li>\r\n \t
  3. Know the major research designs that can be used to study nature\u2013nurture questions.<\/li>\r\n \t
  4. Appreciate the complexities of nature\u2013nurture and why questions that seem simple turn out not to have simple answers.<\/li>\r\n<\/ol>\r\n<\/div>\r\n \r\n\r\n<\/section>
    \r\n

    Introduction<\/h1>\r\nThere are three related problems at the intersection of philosophy and science that are fundamental to our understanding of our relationship to the natural world: the mind\u2013body problem, the free will problem, and the nature\u2013nurture problem. These great questions have a lot in common. Everyone, even those without much knowledge of science or philosophy, has opinions about the answers to these questions that come simply from observing the world we live in. Our feelings about our relationship with the physical and biological world often seem incomplete. We are in control of our actions in some ways, but at the mercy of our bodies in others; it feels obvious that our consciousness is some kind of creation of our physical brains, at the same time we sense that our awareness must go beyond just the physical. This incomplete knowledge of our relationship with nature leaves us fascinated and a little obsessed, like a cat that climbs into a paper bag and then out again, over and over, mystified every time by a relationship between inner and outer that it can see but can\u2019t quite understand.\r\n\r\nIt may seem obvious that we are born with certain characteristics while others are acquired, and yet of the three great questions about humans\u2019 relationship with the natural world, only nature\u2013nurture gets referred to as a \u201cdebate.\u201d In the history of psychology, no other question has caused so much controversy and offense: We are so concerned with nature\u2013nurture because our very sense of moral character seems to depend on it. While we may admire the athletic skills of a great basketball player, we think of his height as simply a gift, a payoff in the \u201cgenetic lottery.\u201d For the same reason, no one blames a short person for his height or someone\u2019s congenital disability on poor decisions: To state the obvious, it\u2019s \u201cnot their fault.\u201d But we do praise the concert violinist (and perhaps her parents and teachers as well) for her dedication, just as we condemn cheaters, slackers, and bullies for their bad behavior.\r\n\r\nThe problem is, most human characteristics aren\u2019t usually as clear-cut as height or instrument-mastery, affirming our nature\u2013nurture expectations strongly one way or the other. In fact, even the great violinist might have some inborn qualities\u2014perfect pitch, or long, nimble fingers\u2014that support and reward her hard work. And the basketball player might have eaten a diet while growing up that promoted his genetic tendency for being tall. When we think about our own qualities, they seem under our control in some respects, yet beyond our control in others. And often the traits that don\u2019t seem to have an obvious cause are the ones that concern us the most and are far more personally significant. What about how much we drink or worry? What about our honesty, or religiosity, or sexual orientation? They all come from that uncertain zone, neither fixed by nature nor totally under our own control.\r\n
    \"Two
    Researchers have learned a great deal about the nature-nurture dynamic by working with animals. But of course many of the techniques used to study animals cannot be applied to people. Separating these two influences in human subjects is a greater research challenge. [Image: Sebasti\u00e1n Dario, https:\/\/goo.gl\/OPiIWd, CC BY-NC 2.0, https:\/\/goo.gl\/FIlc2e]<\/figcaption><\/figure>\r\nOne major problem with answering nature-nurture questions about people is, how do you set up an experiment? In nonhuman animals, there are relatively straightforward experiments for tackling nature\u2013nurture questions. Say, for example, you are interested in aggressiveness in dogs. You want to test for the more important determinant of aggression: being born to aggressive dogs or being raised by them. You could mate two aggressive dogs\u2014angry Chihuahuas\u2014together, and mate two nonaggressive dogs\u2014happy beagles\u2014together, then switch half the puppies from each litter between the different sets of parents to raise. You would then have puppies born to aggressive parents (the Chihuahuas) but being raised by nonaggressive parents (the Beagles), and vice versa, in litters that mirror each other in puppy distribution. The big questions are: Would the Chihuahua parents raise aggressive beagle puppies? Would the beagle parents raise non<\/em>aggressive Chihuahua puppies? Would the puppies\u2019 nature<\/em>win out, regardless of who raised them? Or... would the result be a combination of nature and<\/em>nurture? Much of the most significant nature\u2013nurture research has been done in this way (Scott & Fuller, 1998<\/a>), and animal breeders have been doing it successfully for thousands of years. In fact, it is fairly easy to breed animals for behavioral traits.\r\n\r\nWith people, however, we can\u2019t assign babies to parents at random, or select parents with certain behavioral characteristics to mate, merely in the interest of science (though history does include horrific examples of such practices, in misguided attempts at \u201ceugenics,\u201d the shaping of human characteristics through intentional breeding). In typical human families, children\u2019s 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\u2014though they only provide partial answers to our many questions.\r\n\r\nThe science of how genes and environments work together to influence behavior is calledbehavioral genetics<\/a>. The easiest opportunity we have to observe this is the adoption study<\/a>. When children are put up for adoption, the parents who give birth to them are no longer the parents who raise them. This setup isn\u2019t quite the same as the experiments with dogs (children aren\u2019t 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\u2019s 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?\r\n
    \"Twin
    Studies focused on twins have led to important insights about the biological origins of many personality characteristics.<\/figcaption><\/figure>\r\nAnother option for observing nature-nurture in humans involves twin studies<\/a>. There are two types of twins: monozygotic (MZ) and dizygotic (DZ). Monozygotic twins, also called \u201cidentical\u201d twins, result from a single zygote (fertilized egg) and have the same DNA. They are essentially clones. Dizygotic twins, also known as \u201cfraternal\u201d twins, develop from two zygotes and share 50% of their DNA. Fraternal twins are ordinary siblings who happen to have been born at the same time. To analyze nature\u2013nurture using twins, we compare the similarity of MZ and DZ pairs. Sticking with the features of height and spoken language, let\u2019s 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\u2019t make much difference.\r\n\r\nTwin and adoption studies are two instances of a much broader class of methods for observing nature-nurture called quantitative genetics<\/a>, 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, twins who have been separated at birth and raised separately (Bouchard, Lykken, McGue, & Segal, 1990<\/a>; such twins are very rare and play a smaller role than is commonly believed in the science of nature\u2013nurture), or with entire extended families (see Plomin, DeFries, Knopik, & Neiderhiser, 2012<\/a>, for a complete introduction to research methods relevant to nature\u2013nurture).\r\n\r\nFor better or for worse, contentions about nature\u2013nurture have intensified because quantitative genetics produces a number called a heritability coefficient<\/a>, varying from 0 to 1, that is meant to provide a single measure of genetics\u2019 influence of a trait. In a general way, a heritability coefficient measures how strongly differences among individuals are related to differences among their genes. But beware: Heritability coefficients, although simple to compute, are deceptively difficult to interpret. Nevertheless, numbers that provide simple answers to complicated questions tend to have a strong influence on the human imagination, and a great deal of time has been spent discussing whether the heritability of intelligence or personality or depression is equal to one number or another.\r\n
    \"A
    Quantitative genetics uses statistical methods to study the effects that both heredity and environment have on test subjects. These methods have provided us with the heritability coefficient which measures how strongly differences among individuals for a trait are related to differences among their genes. [Image: EMSL, https:\/\/goo.gl\/IRfn9g, CC BY-NC-SA 2.0, https:\/\/goo.gl\/fbv27n]<\/figcaption><\/figure>\r\nOne reason nature\u2013nurture continues to fascinate us so much is that we live in an era of great scientific discovery in genetics, comparable to the times of Copernicus, Galileo, and Newton, with regard to astronomy and physics. Every day, it seems, new discoveries are made, new possibilities proposed. When Francis Galton first started thinking about nature\u2013nurture in the late-19th century he was very influenced by his cousin, Charles Darwin, but genetics per se<\/em> was unknown. Mendel\u2019s famous work with peas, conducted at about the same time, went undiscovered for 20 years; quantitative genetics was developed in the 1920s; DNA was discovered by Watson and Crick in the 1950s; the human genome was completely sequenced at the turn of the 21st century; and we are now on the verge of being able to obtain the specific DNA sequence of anyone at a relatively low cost. No one knows what this new genetic knowledge will mean for the study of nature\u2013nurture, but as we will see in the next section, answers to nature\u2013nurture questions have turned out to be far more difficult and mysterious than anyone imagined.\r\n

    What Have We Learned About Nature\u2013Nurture?<\/h1>\r\nIt would be satisfying to be able to say that nature\u2013nurture 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 childrearing practices and personal will; but that is not the case. Instead, everything<\/em> has turned out to have some footing in genetics. The more genetically-related people are, the more similar they are\u2014foreverything<\/em>: 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<\/a>), and whether or not they get divorced (McGue & Lykken, 1992<\/a>).\r\n
    \"A
    Research over the last half century has revealed how central genetics are to behavior. The more genetically related people are the more similar they are not just physically but also in terms of personality and behavior. [Image: Paul Altobelli, https:\/\/goo.gl\/SWLwm2, CC BY 2.0, https:\/\/goo.gl\/9uSnqN]<\/figcaption><\/figure>\r\nIt may seem surprising, but genetic influence on behavior is a relatively recent discovery. In the middle of the 20th century, psychology was dominated by the doctrine of behaviorism, which held that behavior could only be explained in terms of environmental factors. Psychiatry concentrated on psychoanalysis, which probed for roots of behavior in individuals\u2019 early life-histories. The truth is, neither behaviorism nor psychoanalysis is incompatible with genetic influences on behavior, and neither Freud nor Skinner was naive about the importance of organic processes in behavior. Nevertheless, in their day it was widely thought that children\u2019s personalities were shaped entirely by imitating their parents\u2019 behavior, and that schizophrenia was caused by certain kinds of \u201cpathological mothering.\u201d Whatever the outcome of our broader discussion of nature\u2013nurture, the basic fact that the best predictors of an adopted child\u2019s personality or mental health are found in the biological parents he or she has never met, rather than in the adoptive parents who raised him or her, presents a significant challenge to purely environmental explanations of personality or psychopathology. The message is clear: You can\u2019t leave genes out of the equation. But keep in mind, no behavioral traits are completely inherited, so you can\u2019t leave the environment out altogether, either.\r\n\r\nTrying to untangle the various ways nature-nurture influences human behavior can be messy, and often common-sense notions can get in the way of good science. One very significant contribution of behavioral genetics that has changed psychology for good can be very helpful to keep in mind: When your subjects are biologically-related, no matter how clearly a situation may seem to point to environmental influence, it is never safe to interpret a behavior as wholly the result of nurture without further evidence. For example, when presented with data showing that children whose mothers read to them often are likely to have better reading scores in third grade, it is tempting to conclude that reading to your kids out loud is important to success in school; this may well be true, but the study as described is inconclusive, because there are genetic as well as<\/em> environmental pathways between the parenting practices of mothers and the abilities of their children. This is a case where \u201ccorrelation does not imply causation,\u201d as they say. To establish that reading aloud causes success, a scientist can either study the problem in adoptive families (in which the genetic pathway is absent) or by finding a way to randomly assign children to oral reading conditions.\r\n\r\nThe outcomes of nature\u2013nurture studies have fallen short of our expectations (of establishing clear-cut bases for traits) in many ways. The most disappointing outcome has been the inability to organize traits from more<\/em>- to less<\/em>-genetic. As noted earlier, everything has turned out to be at least somewhat<\/em> heritable (passed down), yet nothing has turned out to be absolutely<\/em>heritable, and there hasn\u2019t been much consistency as to which traits are more<\/em> heritable and which are less<\/em> heritable once other considerations (such as how accurately the trait can be measured) are taken into account (Turkheimer, 2000<\/a>). The problem is conceptual: The heritability coefficient, and, in fact, the whole quantitative structure that underlies it, does not match up with our nature\u2013nurture intuitions. We want to know how \u201cimportant\u201d the roles of genes and environment are to the development of a trait, but in focusing on \u201cimportant\u201d maybe we\u2019re emphasizing the wrong thing. First of all, genes and environment are both crucial to every<\/em> trait; without genes the environment would have nothing to work on, and too, genes cannot develop in a vacuum. Even more important, because nature\u2013nurture questions look at the differences among people, the cause of a given trait depends not only on the trait itself, but also on the differences in that trait between members of the group being studied.\r\n\r\nThe classic example of the heritability coefficient defying intuition is the trait of having two arms. No one would argue against the development of arms being a biological, genetic process. But fraternal twins are just as similar for \u201ctwo-armedness\u201d as identical twins, resulting in a heritability coefficient of zero for the trait of having two arms. Normally, according to the heritability model, this result (coefficient of zero) would suggest all nurture, no nature, but we know that\u2019s not the case. The reason this result is not a tip-off that arm development is less genetic than we imagine is because people do not vary<\/em> in the genes related to arm development\u2014which essentially upends the heritability formula. In fact, in this instance, the opposite is likely true: the extent that people differ in arm number is likely the result of accidents and, therefore, environmental. For reasons like these, we always have to be very careful when asking nature\u2013nurture questions, especially when we try to express the answer in terms of a single number. The heritability of a trait is not simply a property of that trait, but a property of the trait in a particular context of relevant genes and environmental factors.\r\n\r\nAnother issue with the heritability coefficient is that it divides traits\u2019 determinants into two portions\u2014genes and environment\u2014which are then calculated together for the total variability. This is a little like asking how much of the experience of a symphony comes from the horns and how much from the strings; the ways instruments or genes integrate is more complex than that. It turns out to be the case that, for many traits, genetic differences affect behavior under some environmental circumstances but not others\u2014a phenomenon called gene-environment interaction, or G x E. In one well-known example, Caspi et al. (2002<\/a>) showed that among maltreated children, those who carried a particular allele of the MAOA gene showed a predisposition to violence and antisocial behavior, while those with other alleles did not. Whereas, in children who had not been maltreated, the gene had no effect. Making matters even more complicated are very recent studies of what is known as epigenetics (see module, \u201cEpigenetics\u201d http:\/\/noba.to\/37p5cb8v), a process in which the DNA itself is modified by environmental events, and those genetic changes transmitted to children.\r\n
    \"A
    The answer to the nature \u2013nurture question has not turned out to be as straightforward as we would like. The many questions we can ask about the relationships among genes, environments, and human traits may have many different answers, and the answer to one tells us little about the answers to the others. [Image: Sundaram Ramaswamy, https:\/\/goo.gl\/Bv8lp6, CC BY 2.0, https:\/\/goo.gl\/9uSnqN]<\/figcaption><\/figure>\r\nSome common questions about nature\u2013nurture are, how susceptible is a trait to change, how malleable is it, and do we \u201chave a choice\u201d about it? These questions are much more complex than they may seem at first glance. For example, phenylketonuria is an inborn error of metabolism caused by a single gene; it prevents the body from metabolizing phenylalanine. Untreated, it causes mental retardation and death. But it can be treated effectively by a straightforward environmental intervention: avoiding foods containing phenylalanine. Height seems like a trait firmly rooted in our nature and unchangeable, but the average height of many populations in Asia and Europe has increased significantly in the past 100 years, due to changes in diet and the alleviation of poverty. Even the most modern genetics has not provided definitive answers to nature\u2013nurture questions. When it was first becoming possible to measure the DNA sequences of individual people, it was widely thought that we would quickly progress to finding the specific genes that account for behavioral characteristics, but that hasn\u2019t happened. There are a few rare genes that have been found to have significant (almost always negative) effects, such as the single gene that causes Huntington\u2019s disease, or the Apolipoprotein gene that causes early onset dementia in a small percentage of Alzheimer\u2019s cases. Aside from these rare genes of great effect, however, the genetic impact on behavior is broken up over many genes, each with very small effects. For most behavioral traits, the effects are so small and distributed across so many genes that we have not been able to catalog them in a meaningful way. In fact, the same is true of environmental effects. We know that extreme environmental hardship causes catastrophic effects for many behavioral outcomes, but fortunately extreme environmental hardship is very rare. Within the normal range of environmental events, those responsible for differences (e.g., why some children in a suburban third-grade classroom perform better than others) are much more difficult to grasp.\r\n\r\nThe difficulties with finding clear-cut solutions to nature\u2013nurture problems bring us back to the other great questions about our relationship with the natural world: the mind-body problem and free will. Investigations into what we mean when we say we are aware of something reveal that consciousness is not simply the product of a particular area of the brain, nor does choice turn out to be an orderly activity that we can apply to some behaviors but not others. So it is with nature and nurture: What at first may seem to be a straightforward matter, able to be indexed with a single number, becomes more and more complicated the closer we look. The many questions we can ask about the intersection among genes, environments, and human traits\u2014how sensitive are traits to environmental change, and how common are those influential environments; are parents or culture more relevant; how sensitive are traits to differences in genes, and how much do the relevant genes vary in a particular population; does the trait involve a single gene or a great many genes; is the trait more easily described in genetic or more-complex behavioral terms?\u2014may have different answers, and the answer to one tells us little about the answers to the others.\r\n\r\nIt is tempting to predict that the more we understand the wide-ranging effects of genetic differences on all human characteristics\u2014especially behavioral ones\u2014our cultural, ethical, legal, and personal ways of thinking about ourselves will have to undergo profound changes in response. Perhaps criminal proceedings will consider genetic background. Parents, presented with the genetic sequence of their children, will be faced with difficult decisions about reproduction. These hopes or fears are often exaggerated. In some ways, our thinking may need to change\u2014for example, when we consider the meaning behind the fundamental American principle that all men are created equal. Human beings differ, and like all evolved organisms they differ genetically. The Declaration of Independence predates Darwin and Mendel, but it is hard to imagine that Jefferson\u2014whose genius encompassed botany as well as moral philosophy\u2014would have been alarmed to learn about the genetic diversity of organisms. One of the most important things modern genetics has taught us is that almost all human behavior is too complex to be nailed down, even from the most complete genetic information, unless we\u2019re looking at identical twins. The science of nature and nurture has demonstrated that genetic differences among people are vital to human moral equality, freedom, and self-determination, not opposed to them. As Mordecai Kaplan said about the role of the past in Jewish theology, genetics gets a vote, not a veto, in the determination of human behavior. We should indulge our fascination with nature\u2013nurture while resisting the temptation to oversimplify it.\r\n\r\n<\/section>
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    Exercises<\/h3>\r\n

    Take a Quiz<\/h2>\r\n
    <\/form>Testing yourself regularly is one of the most effective ways to strengthen your learning. Frequent testing helps you identify what you know and don\u2019t know so you can allocate your study time wisely. It also helps you retain information in memory for longer periods of time.\r\n\r\nBelow you will find a link to a 20-item quiz covering the main concepts found in this module. We suggest you start by learning 10 items. When the first session is complete you can either learn the final 10 items in a new session, review items from the first session, or return later.\r\n\r\nTo begin the quiz, click the \"Start Learning\" button.<\/strong> You can return to this quiz anytime to refresh your knowledge.\r\n\r\n<\/div>\r\n \r\n\r\n<\/section>
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    Outside Resources<\/h3>\r\n
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    Web: Institute for Behavioral Genetics<\/dt>\r\n \t
    http:\/\/www.colorado.edu\/ibg\/<\/a><\/dd>\r\n<\/dl>\r\n<\/div>\r\n \r\n
    <\/dl>\r\n<\/section>
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    Discussion Questions<\/h3>\r\n
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    1. Is your personality more like one of your parents than the other? If you have a sibling, is his or her personality like yours? In your family, how did these similarities and differences develop? What do you think caused them?<\/li>\r\n \t
    2. Can you think of a human characteristic for which genetic differences would play almost no role? Defend your choice.<\/li>\r\n \t
    3. Do you think the time will come when we will be able to predict almost everything about someone by examining their DNA on the day they are born?<\/li>\r\n \t
    4. Identical twins are more similar than fraternal twins for the trait of aggressiveness, as well as for criminal behavior. Do these facts have implications for the courtroom? If it can be shown that a violent criminal had violent parents, should it make a difference in culpability or sentencing?<\/li>\r\n<\/ol>\r\n<\/div>\r\n \r\n\r\n<\/section>
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      Vocabulary<\/h3>\r\n
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      Adoption study<\/strong><\/dt>\r\n \t
      A behavior genetic research method that involves comparison of adopted children to their adoptive and biological parents.<\/dd>\r\n \t
      <\/dd>\r\n \t
      <\/dd>\r\n \t
      Behavioral genetics<\/strong><\/dt>\r\n \t
      The empirical science of how genes and environments combine to generate behavior.<\/dd>\r\n \t
      <\/dd>\r\n \t
      <\/dd>\r\n \t
      Heritability coefficient<\/strong><\/dt>\r\n \t
      An easily misinterpreted statistical construct that purports to measure the role of genetics in the explanation of differences among individuals.<\/dd>\r\n \t
      <\/dd>\r\n \t
      <\/dd>\r\n \t
      Quantitative genetics<\/strong><\/dt>\r\n \t
      Scientific and mathematical methods for inferring genetic and environmental processes based on the degree of genetic and environmental similarity among organisms.<\/dd>\r\n \t
      <\/dd>\r\n \t
      <\/dd>\r\n \t
      Twin studies<\/strong><\/dt>\r\n \t
      A behavior genetic research method that involves comparison of the similarity of identical (monozygotic; MZ) and fraternal (dizygotic; DZ) twins.<\/dd>\r\n<\/dl>\r\n<\/div>\r\n \r\n\r\n<\/section>
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      References<\/h2>\r\n