Psychologists agree that environmental factors interact with genetic factors to form personality. Some psychologists have proposed theories that emphasize these genetic influences on personality.
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.