Personality Traits

The most commonly studied personality traits in behavioral genetic designs have been extraversion and neuroticism. Recall that extraversion is a dimension containing people who are outgoing and talkative at one end and people who are quiet and withdrawn at the other (introverted) end. Neuroticism is a dimension with one end characterized by people who tend to be anxious, nervous, and emotionally volatile and the other end having people who tend to be calm and emotionally stable. Henderson (1982) reviewed the literature on more than 25,000 pairs of twins. He found substantial heritability for both traits. In one study involving 4,987 twin pairs in Sweden, for example, the correlations for extraversion were +.51 for identical twins and + .21 for fraternal twins (Floderus-Myrhed, Pedersen, & Rasmuson, 1980). Using the simple rule-of-thumb formula of doubling the dif ference between the two correlations yields a heritability of .60.

The findings for neuroticism were similar (Floderus-Myrhed et al., 1980). The identical twin correlation for neuroticism was + .50, whereas the fraternal twin correlation was only + .23. This suggests a heritability of .54. Twin studies have yielded very similar results, suggesting that extraversion and neuroticism are traits that are approximately half due to genetics. The most recent large-scale twin study , conducted in Australia, found a heritability for neuroticism of 47 percent (Birley, Gillespie, Heath, Sullivan, Boomsma, & Martin, 2006).

The findings for extraversion and neuroticis from adoption studies suggest somewhat lower heri-tabilities. Pedersen (1993), for example, found heritability estimates based on comparisons of adoptees and their biological parents of about 40 percent for extraversion and about 30 percent for neuroticism. Correlations between adoptive parents and their adopted children tend to be around zero, suggesting little direct environmental influence on these traits

Individual differences in activity level have also been subjected to behavioral genetic analysis. You may recall from Chapter 5 that individual dif ferences activity level, measured with a mechanical recording device called an "actometer," emerges early in life and show stability in children over time. Recently , activity level was assessed in an adult sample of 300 monozygotic and dizygotic twin pairs residing in Germany (Spinath, Wolf, Angleitner, Borkenau, & Riemann, 2002). The The trait of activity level—how vigorous and energetic a person researchers measured the physical ener gy each indi-is—shows a moderate degree of heritability. vidual expended through body movements, recorded

mechanically with motion recorders analogous to self-winding wristwatches. Movement of a person's limbs activates the device, which records the frequency and intensity of body activity . Activity level showed a heritability of .40, suggesting that a moderate proportion of the individual dif ferences in motor ener gy expended are due to genetic differences.

Activity level is one among several temperaments that show moderate heritability. A study of 1,555 twins in Poland found 50 percent heritability , on average, for all temperaments, including activity, emotionality, sociability, persistence, fear, and distractibility (Oniszczenko et al., 2003). A study of Dutch twins, at ages 3, 7, and 10, found even higher heritabilities for aggressiveness, ranging from 51 to 72 percent (Hudziak, van Beijsterveldt, Bartels, Rietvelt, Rettew , Derks, & Boomsma, 2003).

Behavioral genetic studies have also been carried out on a wide array of other personality dispositions. Using 353 male twins from the Minnesota Twin Registry, researchers explored the heritability of so-called "psychopathic" personality traits (Blonigen, Carlson, Krueger , & Patrick, 2003). These include traits such as Machiavellianism (e.g., enjoys manipulating other people), Coldheartedness (e.g., has a callous emotional style), Impulsive Nonconformity (e.g., indif ferent to social conventions), Fearlessness (e.g., a risk taker; lacks anticipatory anxiety concerning harm), Blame Externalization (e.g., blames others for one' s problems), and Stress Immunity (e.g., lacks anxiety when faced with stressful life events). All of these "psychopathic" personality traits showed moderate to high heritability . For example, for Coldheart-edness, the rmz was +.34, whereas the rdz was -.16; for Fearlessness, the rmz was + .54, whereas the rdz was only .03. Using the method of doubling the dif ference between the MZ and DZ correlations suggests substantial heritability to all of these psychopathic-related personality dispositions.

Interestingly, heritability of personality might not be limited to our own species. In an innovative study of chimpanzees, Weiss, King, and Enns (2002) explored the heritability of dominance (high extraversion, low neuroticism) and well-being (e.g., seems happy, contented, and enjoying itself), as indexed by trained observer judgments. Individual differences in chimpanzee well-being showed a moderate heritabil-ity of .40, whereas individual dif ferences in chimpanzee dominance showed an even stronger heritability of .66. These findings suggest that the importance of genes i influencing personality may not be restricted to humans, but instead may extend t other primates.

Behavioral genetic studies using more comprehensive personality inventories have also been carried out in many different countries as personality research expands to include more and more cross-cultural work. A study of 296 twin pairs in Japan revealed moderate heritability for Cloninger' s Seven-Factor model of temperament and character, which includes dispositions such as novelty seeking, harm avoidance, reward dependence, and persistence (Ando, Ono, Yoshimura, Onoda, Shinohara, Kanba, & Asai, 2002). A study of 168 MZ and 132 DZ twins in Germany, using observational methodology, revealed a 40 percent heritability to markers of the Big Five (Borkenau, Reimann, Angleitner, & Spinath, 2001). Similar findings for the Big Fiv personality traits have been documented in Canada and Germany using self-report measures (Jang, Livesley, Angleitner, Reimann, & Vernon, 2002).

Perhaps the most fascinating study to examine personality traits is the Minnesota Twin Study (Bouchard & McGue, 1990; Tellegen et al., 1988). This study examined 45 sets of identical twins reared apart and 26 sets of fraternal twins reared apart. The researchers found the correlations shown in T able 6.2 between identical twins reared apart. These findings startled many people. How could traditionalism

Personality Trait

Twin Correlation

Sense of well-being Social potency Achievement orientation Social closeness Neuroticism Sense of alienation Aggression Inhibited control Low risk taking Traditionalism Absorption or imagination Average twin correlation

Personality Trait

Twin Correlation

Sense of well-being Social potency Achievement orientation Social closeness Neuroticism Sense of alienation Aggression Inhibited control Low risk taking Traditionalism Absorption or imagination Average twin correlation

Sources: Bouchard & McGue, 1990; Tellegen et al., 1988.

for example, which reflects an attitude or a preference for the established ways o doing things, show such strong heritability? And how could neuroticism have such a high heritability, given the traditional view that it is parents who make their children neurotic by their inconsistency of reinforcement and improper attachment? These behavioral genetic findings caused some researchers to question long-hel assumptions about the origins of individual dif ferences—a topic we will consider later in this chapter under the heading "Shared versus Nonshared Environmental Influences: Riddle."

Summaries of the behavioral genetic data for many of the major personality traits— extraversion, agreeableness, conscientiousness, neuroticism, openness to experience— yield heritability estimates of approximately 50 percent (Bouchard & Loehlin, 2001; Caspi, Roberts, & Shiner , 2005). Furthermore, it is clear that the heritability of personality is heavily responsible for the fact that personality traits remain fairly stable over time (Blonigen et al., 2006; Caspi, Roberts, & Shiner , 2005; Johnson, McGue, & Krueger, 2005; van Beijsterveldt, Bartels, Hudziak, & Boomsma, 2003). Overall, it is clear that major personality traits show a modest degree of heritability , at least for the samples that have been studied so far . The same studies, however , also suggest that a substantial portion of the variance in personality traits is environmental in origin.

Stable attitudes are generally regarded to be part of personality—they show wide individual differences, they tend to be stable over time, and at least sometimes they are linked with actual behavior . Behavioral geneticists have also examined the heritability of attitudes. The Minnesota Twin Study showed that traditionalism—as evidenced by attitudes favoring conservative values over modern values—showed a heritability of .63. One study of more than 2,000 twin pairs living in Australia found

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Anxiety and Depression 101

Anxiety and Depression 101

Everything you ever wanted to know about. We have been discussing depression and anxiety and how different information that is out on the market only seems to target one particular cure for these two common conditions that seem to walk hand in hand.

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