Psychologist Hans Eysenck believes that genetics are the primary determinate of personality, although he thinks conditioning also plays a role. According to Eysenck, personality traits are hierarchical, with a few basic traits giving rise to a large array of more superficial traits. Genetically determined differences in physiological functioning make some people more vulnerable to behavioral conditioning. Eysenck suggests that introverted people have higher levels of physiological arousal, which allows them to be conditioned by environmental stimuli more easily. Because of this, such people develop more inhibitions, which make them more shy and uneasy in social situations.

Empirical evidence for genetic contributions to personality comes mainly from two kinds of studies: studies of children’s temperaments and heritability studies.

Temperament refers to innate personality features or dispositions. Babies show particular temperaments soon after birth. Temperaments that researchers have studied include reactivity, which refers to a baby’s excitability or responsiveness, and soothability, which refers to the ease or difficulty of calming an upset baby.

Researchers have studied children from infancy to adolescence and found that temperaments remain fairly stable over time. However, temperaments can also be modified over time by environmental factors.

Heritability studies also provide evidence for genetic contributions to personality. Heritability is a mathematical estimate that indicates how much of a trait’s variation in a population can be attributed to genes. For more information about heritability, see page 35.

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.