This chapter begins with a review of some of the definitions of the trait of extraversion found in Chapter 1. Eysenck (1947, 1967) traced the history of the introversion-extraversion construct back to Galen's 2nd-century types. These types were based on the “humors” of the body: phlegmatic, sanguine, melancholic, and choleric. The ancient Greeks regarded these as pure types rather than bipolar traits. In the 19th century, psychiatrists described the types in terms of psychopathologies: schizophrenic versus manic-depressive (Kraepelin, 1899) and psychasthenia versus hysteria (Janet, 1907), regarding the abnormal manifestations as the extremes of normal personality types. Jung (1933) used the terms introversion and extraverson to describe what he believed to be the major dimension of personality: an orientation to the external world of people and objects versus a primary direction of interest and motive to the internal world. He described extreme types, but he recognized that “the normal man is, by definition, influenced as much from within as without,” implying a continuum or normal dimension of personality. Freud (1920), in contrast to Jung, regarded introversion as a precursor of neurosis. The association between introversion and neuroticism was reflected in some of the early American questionnaires that showed high correlations between introversion and neuroticism. The idea of a normal introvert was not widely accepted in the extraverted American culture.

Eysenck's construct was closer to Jung's, in that he regarded introversion extraversion as a normal dimension independent of and uncorrelated with the major dimension of neuroticism.