Extraversion-introversion and emotional stability-instability
Hans Eysenck (1916-1997) was the one who received more acclaim for his work in the field of psychology, but his wife Sybil (now his widow) is a personality psychologist also and collaborated much with her husband.
The terms extraversion and introversion were first popularized by Carl Jung. However, Hans Eysenck did much further development of the extraversion-introversion scale in terms of how individuals interact with other people.
The Eysenck Personality Questionnaire also includes emotional stability-instability as another key factor in personality. The questionnaire yields results that classify persons as either stable introverts, unstable introverts, stable extraverts, or unstable extraverts.