Prepared for the Handbook of Individual Differences in Social Behavior

For at least 2500 years, some people have been described as more bold, assertive and talkative than others. For almost equally long, this set of behaviors has been thought to have a biological basis and be socially important. Although our taxometric techniques have changed and our theories of biology are more advanced, the question of the causal basis as well as the behavioral consequences of the trait dimension that has come to be called extraversion-introversion1 remains vitally important. In general, there are at least three basic characteristics of extraversion that make it important to study. First, extraversion has emerged as one of the fundamental dimensions of personality (Costa & McCrae, 1992a; Digman, 1990; H. J. Eysenck & Himmelweit, 1947; Goldberg, 1990; Norman, 1963). As such, extraversion has the potential to explain the covariation of a wide variety of behaviors, which is is one of the central concerns for the field of personality (Funder, 2001). Second, extraversion predicts effective functioning and well-being across a wide variety of domains (Ozer & Benet-Martinez, 2006) from cognitive performance (Matthews, 1992) and social endeavors (Eaton & Funder, 2003) to social economic status (Roberts, Kuncel, Shiner, Caspi, & Goldberg, 2007). Third, extraversion predicts risk and also resilience for different forms of psychopathology (Trull & Sher, 1994; Widiger, 2005).