Although trait theories have tried to be largely atheoretical, they all assume a similar underlying cause of differences between people--namely, biology. This can be seen in Eysenck's emphasis on differences in excitability in introverts and extroverts as the basis of differences in behavior. In this section, we first examine the evidence for heritability of behavioral traits, focusing particularly on evidence from studies of twins. Next, we discuss the research and theory of Jerome Kagan, who has argued that approximately 20 percent of children are born with an "inhibited" temperament.

The concept of heritability is central to behavioral genetics, the study of the effect of genes on behavior. Heritability refers to the extent to which a particular behavior or pattern of behaviorism (e.g. aggression, intelligence) is influenced by genes. If every child of two equally aggressive parents was as aggressive as his or her parents, regardless of the environment in which the child was raised, then aggressiveness would have a heritability of 1.00. In real life, no behavioral trait has a heritability greater than 0.50; that is, genes do not account for more than half the variance among people on any given behavioral trait. In order to determine the heritability of a trait, researchers turn to studies of the families of people with a particular trait and to studies of twins. The most rigorous of these kinds of studies compared identical twins that have been reared in separate households (because of adoption) to fraternal twins that have been reared in separate households. These studies have found two surprising things: first, although heritability of a trait is never at 0.50, it is often close to that level, which is surprisingly large, given the evident impact of environment on behavior; second, most of the salient environmental variables (e.g. education, socioeconomic class, parenting style) seem to account for almost none of the variance.

Jerome Kagan has argued that somewhere between 10 and 20 percent of children are born with an "inhibited" temperament that is strongly influenced by genes. Kagan came to this conclusion by studying the ways in which infants and young children respond to strange or novel events. He found that some infants tended to cry and become agitated ("high cry, high motor activity") when presented with such events, whereas most other infants remained calm in such situations ("low cry, low motor activity"). When tested again several years later, the "high cry, high motor" children were much less likely to approach a stranger or to play with a new toy than the "low cry, low motor" children. Kagan concluded that, since the differences appeared very early in life and remained stable for at least several years, they were probably inherited. Kagan has hypothesized that these difference might be due to genetic influences on the sensitivity of the amygdala, a structure in the brain that has been associated with emotional reactions, particularly fear. Kagan has further argued that "inhibited" children are not merely at one extreme of a continuum, but rather that they have inherited a qualitatively different pattern of behaviors than non-inhibited children. He uses this to suggests that a dimensional approach to personality may be less useful than a categorical approach.