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Twin studies help researchers to determine heritability, as described in Chapter 2, “Evolution and Genes.” Researchers have shown that identical twins raised together are more similar than fraternal twins raised together in traits such as positive emotionality, negative emotionality, and constraint. Identical twins separated early in life and raised apart are more similar in these traits than are fraternal twins raised together. Both of these research findings suggest the existence of a genetic component to personality.
Behavioral geneticists have shown, after doing studies in many different countries, that the heritability of personality traits is around .5, which means that 50 percent of the variation in personality traits in a group of people can be attributed to genetic differences among those people.
The environment also has important influences on personality. These include peer relationships and the kinds of situations a child encounters. As described on page 277, under “Walter Mischel’s Ideas,” the interactions between innate characteristics and environmental factors are two-way. Children’s temperaments are likely to influence their peer relationships and the situations they encounter. Similarly, peers and situations can modify children’s personality characteristics.
Evolutionary theorists explain personality in terms of its adaptive value. Theorists such as David Buss have argued that the Big Five personality traits are universally important because these traits have given humans a reproductive advantage.
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