In the other Topics, we have discussed a variety of approaches to personality, each of which starts from a particular theoretical framework--such as the importance of instincts and early life experience in Freudian theory--and then goes on to investigate how people differ within that framework. In this section, we consider an approach that started very differently. Trait theory, unlike the rest of the theories we discuss, is almost completely "atheoretic"; that is, it is based on empirical studies of the ways people describe themselves and their own behavior, and makes few or no claims about the underlying processes that might lead to such behaviors and self-descriptions.

How could one go about creating an atheoretic approach to personality? The early trait theorists, like Raymond Cattell and Hans Eysenck, did it by going to the dictionary. After culling from it a large set of words that could be used to describe someone's personality, they then asked large numbers of people to fill out questionnaires indicating the extent to which they, or other people, could be described with each of those words. Using a statistical method called factor analysis, they then tried to determine whether there were clusters of words that tended to hang together. An example of a group of words that hang together is "happy," "glad," and "joyful": anyone who can be described with one of these words can probably also be described with the other two.

In the first section, we summarize the models that Cattell and Eysenck came up with to describe the results of their factor analyses. In brief, Cattell believed that there were sixteen important factors--for example, "sociable-unsociable" or "shrewd-naive." Eysenck came up with a much simpler model, one that included only two basic traits: "extroversion-introversion" and "neuroticism-stability." Note that both Cattell's and Eysenck's traits describe dimensions along which personality can vary, a personality continuum rather than particular personality types. In this section we also describe the currently accepted set of basic traits, often known as the "Big Five": openness, conscientiousness, extroversion, agreeableness, and neuroticism.

In the second section, we examine one of trait theory's assumptions: that traits are heavily influenced by biology. First we discuss evidence in the field of behavioral genetics derived from studies of adopted children and twins reared in different families. This evidence suggests that many aspects of personality are highly heritable and that the environment may not play as large a role as we think. Second, we discuss Jerome Kagan's theory of temperament, particularly his studies of inhibited children, and the differences and similarities between his approach and the approach of trait theorists.