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3 10 The Biological Approach Relevant Research Heritability of Personality Traits Extraversion–Introversion Evolutionary Personality Theory and Mate Selection DNY59/E+/Getty Images Summary Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or...

3 10 The Biological Approach Relevant Research Heritability of Personality Traits Extraversion–Introversion Evolutionary Personality Theory and Mate Selection DNY59/E+/Getty Images Summary Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 228 Chapter 10 / The Biological Approach T oday most psychologists readily embrace the notion that biology plays a role in human personality, but students are often surprised to hear that this was not always the case. In truth, many psychologists came to accept this conclusion rather reluctantly. Why the resistance? One reason is that the “blank slate” view of humankind has great appeal. If we accept that personality is formed largely or exclusively by experiences, in theory we can mold an individual into whatever kind of person we want. With enough knowledge and resources, we could eliminate low self-esteem, pessimism, neuroticism, and other personality traits that often create difficulties. But accepting that biology plays a role in personality development usually means limiting these possibilities for change. Another reason some psychologists were hesitant to accept the biological approach was a concern about inappropriate and even offensive interpretations that come from placing too much emphasis on biological determinants. In the past, some people have argued against social programs by maintaining that certain racial or gender differences are the result of biological rather than cultural factors. Of course, accepting a biological component to personality does not mean that personality is fixed at birth. Those who resign themselves with “That’s the way men/ women are” or “It’s just my nature” are foolishly ignoring the power of experience. But it would be equally foolish to ignore the wealth of evidence indicating that biology has a hand in shaping personality. The most persuasive case for the biological approach can be found in the growing amount of supportive research findings. We’ll review some of those findings in this chapter. As with research from other approaches to personality, the studies reported here have limitations and are sometimes subject to alternate interpretations. However, taken together the data make it difficult to ignore the importance of biological determinants of personality. We begin by looking at research on the heritability of personality characteristics. More specifically, we examine the methods researchers use to determine how much of our personality is inherited from our parents. As you will see, identifying the precise strength of the genetic component remains elusive. Next we’ll review research generated from Hans Eysenck’s theory of personality. Specifically, we’ll look at some of the differences between extraverts and introverts. This research suggests that your level of extraversion–introversion affects a wide range of behavior, including how happy you are and where you sit in the library. We’ll also examine one application of evolutionary personality theory. According to this theory, for evolutionary reasons, men and women should differ in what they look for in a romantic partner. Heritability of Personality Traits H ow much of your personality is the result of your genetic makeup, and how much is the result of the environment you grew up in? This “nature–nurture” question is one of the oldest and most enduring issues in psychology. Interestingly, people with little or no exposure to research seem to readily accept that both genetic background and experiences are important in shaping personality. Parents often point to personality traits their children “got from me,” but few would deny that the way they raise their Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Heritability of Personality Traits 229 children also plays a large role in what kind of adults the children become. Thus the question is not which of these—genetics or environment—shapes our personalities but rather to what extent and how our personalities are shaped by each. So we might rephrase the question this way: To what degree was the mold for your adult personality already cast by the time you were born? Researchers now agree that relatively stable abilities and aptitudes, such as intelligence and related cognitive abilities, appear to have a large genetic component (Kovas et al., 2013; Plomin, DeFries, Knopik, & Neiderhiser, 2016; Tucker-Drob, Briley, & Harden, 2013). This is not to say that a highly intelligent child cannot be born to relatively unintelligent parents or that a child’s environment plays no role in intellectual development. On the contrary, it appears that both environment and culture influence adult intelligence and other cognitive abilities (Bates, Lewis, & Weiss, 2013; Kan, Wicherts, Dolan, & van der Maas, 2013). Similarly, many psychological disorders appear to be affected by the genes we inherited (Crabbe, 2002; DiLalla, Carey, Gottesman, & Bouchard, 1996; McGue & Christensen, 1997; Rhee & Waldman, 2002). Again, this does not mean people are born to be schizophrenic or depressed. Rather, some people are born with a higher susceptibility to these disorders than are others. What about personality traits? Are people born to be aggressive or extraverted? There is now ample evidence that genetics also influences these and other personality traits. However, collecting good evidence on this issue is not easy, and questions remain about how to interpret the data that are available. Separating Environmental from Genetic Influences Psychologists working on the environment–genetics question have a somewhat different task facing them than those working in other areas of personality research. For technological and ethical reasons, it is not possible to manipulate people’s genes and observe the kind of adults they become. Instead, researchers must rely on less direct means. Like detectives trying to piece together a picture of how we got to where we are, these researchers use innovative and sometimes clever experimental procedures to track down the roots of adult personalities. The most obvious source of information on this question is the similarity of parents and children. Aggressive parents often have aggressive offspring; shy children often come from homes with shy parents. Similarly, we often see brothers who are both outgoing or sisters who are both sensitive and caring. Casual observers look at these relationships and often assume the children inherited these traits from their parents. However, there is an obvious alternative explanation for these similarities. Members of a family not only share genes, but share living environments as well. Siblings’ personalities may be similar because the parents raised them in the same basic manner. Children of introverted parents might become introverted because of the calm and quiet home they grow up in. In most cases, therefore, shared genes and shared environments seem hopelessly confounded. Fortunately, researchers have developed ways to peel one of these influences away from the other. The most popular procedure for separating the role of genetics from the role of environment is the twin-study method. This method takes advantage of a naturally occurring phenomenon: the two types of human twins. Some Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Chapter 10 / The Biological Approach Jerry Burger/Santa Clara University 230 Identical twins not only share physical features but also have similar personalities. Researchers attribute this similarity in part to genetic influences although the extent of genetic influence on personality continues to be debated. twins are monozygotic (MZ); that is, the two babies come from the same fertilized egg. These are the twins who look alike physically, the ones we commonly call identical twins. The important point for researchers is that MZ twins have identical genes. The other type, dizygotic (DZ) twins, comes from different eggs. These two babies, commonly called fraternal twins, are no more alike genetically than any two siblings. The logic behind the twin-study method is illustrated in Figure 10.1. We assume that two same-sex DZ twins and two MZ twins (who are always the same sex) share very similar environments. That is, in studies using this method, twin pairs, regardless of type, are the same age and the same sex and live in the same house under the same rules. Therefore, the extent to which the environment is responsible for their personalities is going to be about the same for both types of twin pairs. However, if there is also a genetic influence on personality, we would expect the MZ twins to be more like each other than are the DZ twins because the MZ twins also have identical genes, but the DZ twins do not. Researchers using the twin-study method give personality trait measures to both members of both kinds of twins. They then look at how similar the twin brothers and sisters are on the traits. If trait scores for the MZ twin pairs are more highly correlated than the scores for the DZ twin pairs, we have evidence for genetic influence on personality. Because the environmental influence is roughly the same for both kinds of twins, it is assumed that the MZ twins are more alike because they also have identical genes. Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. I N THE NE WS Genetics and Intelligence A large amount of research indicates that, as with personality traits and psychological disorders, a significant portion of intelligence is determined by our genetic inheritance. Although at first glance this conclusion hardly seems surprising, it is in fact at the heart of a controversy that flares up periodically among psychologists and those who debate social policy. Several decades ago, psychologist Arthur Jensen (1969) considered the research on intelligence and the finding that Black Americans typically score lower on standard intelligence tests than Whites. He suggested from these observations that Blacks might be genetically less intelligent than Whites. Richard Herrnstein and Charles Murray rekindled the debate in 1994 when they published The Bell Curve: Intelligence and Class Structure in American Life. These psychologists also began by pointing out that intelligence is largely inherited. They argued that any gains from educational intervention programs such as Head Start will be short-lived because a child’s genetically determined aptitude will ultimately determine his or her success. Herrnstein and Murray then touched a social and political nerve when they tied their analysis to the question of race. They argued that if Black Americans on average score lower on IQ tests than White Americans, perhaps efforts to provide educational opportunities for African Americans are a waste of time. A decade later, racist groups seized on the findings of genetics researchers who identified recent (within the past 40,000 years) evolutionary changes in brain-related genes (Regalado, 2006). Because differences were found between European and African samples, these individuals drew the scientifically inappropriate conclusion that the research verified claims of genetically based differences in intelligence among racial groups. In each of these situations, reaction was strong and swift. News analysts, political commentators, and political leaders were quick to challenge the interpretations. Reaction from academic psychologists was equally intense. Not only do the vast majority of psychologists find the suggestion of inherent racial differences in intelligence offensive, but they also maintain that such a conclusion is simply not supported by research findings (Flynn, 1999; Neisser et al., 1996; Nisbett, 2009; Sternberg, Grigorenko, & Kidd, 2005). Moreover, it is incorrect to say that intelligence level is fixed by nature and is not amenable to environmental influence (Nisbett et al., 2012). Psychologists are quick to point out that Black children often grow up in an environment that is less intellectually stimulating than that of the average White family (Zernike, 2000). Indeed, researchers find that Black children adopted by White families of reasonable socioeconomic means develop IQ scores no different from those of adopted White children (Nisbett, 2007). Not surprisingly, as preschool and other educational opportunities have become more available to children of all backgrounds, the gap between White and Black students’ IQ scores has narrowed considerably (Nisbett et al., 2012). Beyond this, critics have raised the issue of culture-bound intelligence tests. They argue that the questions asked on most intelligence tests reflect what White, middle-class Americans consider important. One subtest on the widely used Wechsler intelligence tests asks about general knowledge. The assumption behind these questions is that although all children are exposed to this information, the more intelligent ones will attend to and retain it. But clearly a child growing up in an African American culture is exposed to different information from one growing up in a White, middle-class culture. Because of this problem, many psychologists have been working to develop “culture-free” intelligence tests, and recent versions of the adult and children’s Wechsler tests have been revised to account for some of these concerns. 231 Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 232 Chapter 10 / The Biological Approach Monozygotic (MZ) Twins Dizygotic (DZ) Twins Environment same same Genetics same different Figure 10.1 Twin-Study Research Diagram Twin-study research usually generates correlation tables similar to the one found in Table 10.1. In this example, adult MZ and DZ twin pairs were compared on the Big Five personality traits (Chapter 7). As seen in the table, the MZ twin pairs were more similar than the DZ twin pairs in each case (Riemann, Angleitner, & Strelau, 1997). The data in the table are similar to those obtained in other twin studies looking at different measures of the Big Five dimensions (Borkenau, Riemann, Angleitner, & Spinath, 2001; Jang, Livesley, & Vernon, 1996; Jang, McCrae, Angleitner, Riemann, & Livesley, 1998; Loehlin, McCrae, & Costa, 1998; McCrae, Jang, Livesley, Riemann, & Angleitner, 2001). Numerous investigations have been conducted across many cultures using the twin-study method to examine the role genetics plays in personality. One team of researchers estimated that perhaps 800,000 pairs of twins have participated in these studies (Johnson, Turkheimer, Gottesman, & Bouchard, 2009). The size of the effect varies depending on which traits we examine, but when researchers combine twinstudy results from all these investigations, they find that about 40% of the stability Table 10.1 Correlations from a Twin Study MZ Twins DZ Twins Neuroticism .53 .21 Extraversion .56 .33 Openness .54 .35 Agreeableness .42 .24 Conscientiousness .54 .23 Source: Riemann, Angleitner, and Strelau (1997). Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Heritability of Personality Traits 233 in our adult personalities can be attributed to what we inherit from our parents (Johnson et al., 2009; Krueger & Johnson, 2008; Vukasovic & Bratko, 2015). Other methods for teasing apart genetic and environmental influences also find evidence for genetic influence, but usually not as strong as the twin-study data (Plomin & Caspi, 1999). One example comes from research with adopted children. When children are raised from birth by someone other than their biological parents, genetic and environmental influences are not confounded. Think of a family in which parents raise one child they adopted and one they gave birth to. Which child should have a personality similar to the parents’? If genes are playing a role, we would expect the biological offspring to be more like the parents because that child shares not only the environment but also some genes with the parents. In fact, this is what researchers find (Bezdjian, Baker, & Tuvblad, 2011; Klump, Suisman, Burt, McGue, & Iacono, 2009; Scarr, Webber, Weinberg, & Wittig, 1981). However, calculations with the data from these studies indicate the genetic influence is less than that suggested by the twin-study data. In fact, data from adoption studies suggest that the heritability of personality is about half what the twin-study data suggest (Frisell, Pawitan, ­Langstrom, & Lichtenstein, 2012; Vukasovic & Bratko, 2015). But the adoption situation provides even more opportunities to test the genetic– environmental influence question. What would you expect if you compared the personalities of adopted children with those of their biological mothers? The children have shared no environment with the mothers but are still linked by genes. When the personality scores of adopted children are compared with those of their adoptive parents and their biological mothers, the children look more like the biological mothers, whom they have never known (Loehlin, Willerman, & Horn, 1982, 1987). Although the strength of the relationship is also weaker than that suggested by the twin-study data, these findings provide evidence from yet another source that genetics plays at least some role in the formation of adult personalities. It is also possible to combine the twin-study and adoption situations. Although rare, some researchers have taken advantage of situations in which MZ twins are separated from their parents at birth and in addition are raised in two different households. The twins in these pairs share genes but not environments. These twins are then compared with MZ twins raised in the same household, who share both genes and environments. A summary of the findings from studies using this method is shown in Table 10.2. As you can see, the MZ twins tend to be quite similar to each other regardless of whether they are raised with or separated from their twin brother or sister (Rowe, 1987). The obvious explanation for this similarity is that the twins’ genes shaped their personalities in a similar manner regardless of the environments they grew up in. In summary, investigators have used a variety of clever procedures to separate the influence of genetics on personality from the influence of the environment. The consistency of the findings from so many sources suggests that adult personalities clearly are affected by heredity. However, as we will see in the next section, researchers face a number of challenges when interpreting the results of this research, and determining the precise extent of the genetic influence on personality is still a matter of discussion and debate. Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 234 Chapter 10 / The Biological Approach Table 10.2 Correlations for Twins Raised Apart and Twins Raised Together Identical Twins Raised Apart Identical Twins Raised Together Extraversion .61 .51 Neuroticism .53 .50 Intelligence .72 .86 Source: Rowe, D. C. (1987). Resolving the person-situation debate: Invitation to an interdisciplinary dialogue. American Psychologist, 42, 218–227. Reprinted by permission of the American Psychological Association. Interpreting the Heritability Findings The strongest and most consistent evidence in favor of genetic influence on personality comes from twin-study research. However, researchers using this method make two key assumptions. The first is that twin pairs can be accurately identified as MZ or DZ twins. Many “identical” twins may in fact be DZ twins who look very much alike. Fortunately, biological advances have made this less of a problem than it once was. Today, zygosity can be determined in almost all cases through blood tests. The second assumption presents a bigger problem. Researchers assume that MZ and DZ twins have equally similar environments. However, there is some evidence that MZ twins may share more of their environment than DZ twins (Hoffman, 1991; Lytton, 1977; Scarr & Carter-Saltzman, 1979). That is, identical twins may be treated more alike than are DZ twins. Identical twins are often thought of as one unit—they dress alike, are given identical presents, and so on. DZ twins grow up in similar environments, but they are usually allowed to dress differently, join different clubs, and have different friends. DZ twins may even experience environments that are less similar than those typical for siblings (Hoffman, 1985) because parents may look for and emphasize their differences (e.g., “Terry is the studious one”; “Larry is the troublemaker”). If this were the case, we would have to modify Figure 10.1, which would create problems when interpreting the twin-study findings. We wouldn’t be certain whether the higher correlations between MZ twins were caused by greater genetic similarities or greater environmental similarities. This possibility may explain why data from twin-study research suggest a larger role for genetic influences than is found with other procedures. However, some of these other procedures also rely on questionable assumptions (Evans, Gillespie, & Martin, 2002; Hoffman, 1991; Stoolmiller, 1999). Adoptions are not random events. Families who adopt children are typically older, more affluent, more stable, and without many of the problems found in families that do not adopt. Although separated twins may be placed in different homes, the homes typically selected for placement are very similar. As a result, the environmental influences on personality experienced by one twin may be similar to the environmental influences experienced by his or her sibling. We can also challenge the assumption that parents Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Extraversion – Introversion 235 treat an adopted child the same way they do their biological offspring. It is likely parents have different expectations for adopted children. Because they don’t know the biological parents, adopting parents may have few preconceived ideas about how the child’s personality will unfold. In short, some of the discrepancies between the results of twin studies and studies using other methods might be attributed to methodological issues. However, twin studies might produce higher estimates of heritability for another reason. Research suggests that personality traits aren’t passed down from parents to child in a simple, direct manner. Rather, the inherited part of personality is often the result of a complex combination of more than one gene (Finkel & McGue, 1997; Plomin, Corley, Caspi, Fulker, & DeFries, 1998). That is, the genetic influence of some personality traits may not be seen unless a unique combination of more than one gene is inherited. Researchers refer to these complex influences as nonadditive effects. DZ twins share many genes, but they may not share the exact combination of genes that make up a specific personality trait. However, because MZ twins have identical genes, they also share any unique combinations of genes that come together to influence personality. Thus, nonadditive effects would show up in identical twins but not in fraternal twins. If this is the case, it could explain why twin studies find evidence for a larger genetic influence on personality than studies using other methods. So where does this leave us? Exactly how or how much genes determine our adult personalities remains an open question. Some of the answers to this question may come from new methodological and technological developments (Krueger, South, Johnson, & Iacono, 2008). For example, researchers are beginning to identify connections between personality traits and DNA markers for specific genes (Canli, 2008; Gillespie et al., 2008; McCrae, Scally, Terracciano, Abecasis, & Costa, 2010; van den Berg et al., 2016). But regardless of what future discoveries tell us, at this point it seems foolish to ignore the relatively strong case that genetics has an influence on personality. Extraversion – Introversion I n many ways, extraversion–introversion is the most obvious of the major personality dimensions. You may need to know someone fairly well before realizing how conscientious or open-minded he or she is, and most of us are able to hide our less desirable traits, at least for a while. But even after a short conversation with a new acquaintance, we usually have a good idea about whether we are talking to an extravert or an introvert. Extraversion is probably the first characteristic teachers, job interviewers, and potential dates recognize. One team of researchers found that extraversion was by far the most accurately perceived personality dimension among people who knew each other only online (Marriott & Buchanan, 2014). Extraversion–introversion has also drawn the attention of a large number of researchers. Unfortunately, space allows us to examine only three of the many topics investigators have tied to this personality dimension. First, we’ll connect individual differences in extraversion–introversion to the research covered in the previous section by looking at the evidence for the heritability of this personality variable. Second, Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 236 Chapter 10 / The Biological Approach we’ll look at research examining one of the basic differences between introverts and extraverts postulated by Eysenck: preference for arousal. Third, we’ll address the question: Who is happier: introverts or extraverts? The Heritability of Extraversion If you are an introvert, it’s likely you’ve been given some of the following pieces of advice: “You need to get out more often,” “Why can’t you be more sociable?” or “Loosen up and enjoy yourself a little.” Extraverts have probably heard some of these: “There’s more to life than having fun all the time,” “Can’t you think a little before you do something?” or “Slow down and enjoy life.” In other words, whether you are introverted or extraverted, someone has probably asked you to become more of the other. Even the most extreme extravert can sit still for a few minutes, and the most introverted person you know occasionally cuts loose and has a good time with friends. But is it possible for an extravert to become permanently more introverted? Can you raise your child to be less introverted or more extraverted? The answer to these questions depends on what causes a person to become an extravert or an introvert. Hans Eysenck championed the role of genetics in determining personality. Although no one doubts that experiences play a role, Eysenck pointed to inherited tendencies that guide the development of adult behavior styles of extraversion or introversion. Little evidence for heritability was available when Eysenck first introduced his theory of personality, but today an impressive body of work appears to support him on this point. As described earlier, researchers often use the twin-study method to determine the role of genetics in the development of personality. Consequently, much of the evidence for the heritability of extraversion–introversion comes from research comparing correlations between pairs of MZ twins with correlations between pairs of DZ twins. Studies using this procedure find consistent evidence for a genetic component of extraversion–introversion (Baker & Daniels, 1990; Heath, Neale, Kessler, Eaves, & Kendler, 1992; Kandler, Bleidorn, Reimann, Angleitner, & Spinath, 2011; Kandler et al., 2010; Kandler, Riemann, Spinath, & Angleitner, 2010; van den Berg et al., 2014). Two studies in particular deserve special attention. In one of these, a group of researchers measured extraversion–introversion in 12,898 adult twin pairs in Sweden (Floderus-Myrhed, Pedersen, & Rasmuson, 1980). This number represents virtually all the contactable twins born in Sweden over a 33-year period. Another team of researchers tested 7,144 adult twin pairs in Finland (Rose, Koskenvuo, Kaprio, Sarna, & Langinvainio, 1988). This is nearly every living twin in that country born before 1958. A couple of features of these studies make them particularly noteworthy. First, the samples are large. Second, the participants are composed of nearly every available twin in the designated population. This means researchers don’t have to worry about only a certain kind of person volunteering to participate in the study. When the within-pair correlations for DZ and MZ twins in these samples were compared, considerable evidence for a genetic component for extraversion– introversion was uncovered. As shown in Table 10.3, the MZ twins were more like each other than were the DZ twins. Beyond this, the researchers in the Finnish study Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Extraversion – Introversion Table 10.3 237 Within-Pair Extraversion Correlations for MZ and DZ Twins Males Females MZ Twins DZ Twins MZ Twins DZ Twins Swedish sample .47 .20 .54 .21 Finnish .46 .35 .48 .14 Source: Floderus-Myrhed et al. (1980) and Rose et al. (1988). examined the amount of social contact between the members of the twin pairs as well as the amount of social contact the twins generally engaged in. Although the researchers did find that MZ twins were more likely than DZ twins to stay in communication with each other, this factor alone was not sufficient to explain the differences in MZ and DZ correlations on the extraversion–introversion measure. Another study takes the twin-study method one step further (Pedersen, Plomin, McClearn, & Friberg, 1988). As in the earlier investigations, the researchers compared MZ and DZ twins who grew up together. However, these researchers also located 95 pairs of MZ twins and 220 pairs of DZ twins reared apart. As described earlier, a positive correlation between the scores of identical twins separated at birth and reared in different environments would provide strong evidence for a genetic component. And indeed, as shown in Table 10.4, there was a relatively strong correlation between the scores of MZ twins reared in separate environments, albeit not as strong as that for MZ twins reared together. In sum, extraversion appears to have one of the strongest genetic components of any personality variable studied. This observation makes extraversion–introversion an ideal subject for future studies on the heritability of personality using new methods and technology (Canli, 2006). For example, one team of investigators conducted genome-wide scans on adolescents to see which chromosomes were related to various personality measures (Gillespie et al., 2008). They found links between extraversion and chromosomes 2, 3, 8, and 12. Table 10.4 Within-Pair Correlations of Extraversion Scores for Twins Reared Apart and Together Twins Reared Apart Twins Reared Together MZ Twins DZ Twins MZ Twins DZ Twins .30 .04 .54 .06 Source: Pedersen, N. L., Plomin, R., McClearn, G. E., and Friberg, L. (1988).Neuroticism, extraversion, and related traits in adult twins reared apart and reared together. Journal of Personality and Social Psychology, 55, 950–957. Reprinted by permission of the American Psychological Association. Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.