A Brief History about Introversion
Extraversion and introversion were popularised by Swiss psychoanalyst Carl Jung (1875-1961) in 1921. In Psychological Types, Jung described how extraverts engage with external stimuli. He believed that extraverts direct their energy outwards - towards other people - and gain energy from such encounters. Introverts, meanwhile, focus their energy inwards, towards more solitary, thoughtful activities.
Hans Eysenck, a German-born psychologist who spent much of his life studying personality at the University of London, claimed that extraversion was a key dimension in human personality. Eysenck developed a model of personality, focussing on extraversion and neuroticism traits. After studying his subjects, Eysenck concluded that people’s personalities could be understood using introversion-extraversion and emotionally stable-neuroticism scales.
Eysenck’s wife, Sybil, was a fellow personality psychologist, and together they extended his original model. They later added the psychoticism trait to create the PEN model of personality. According to this model, extraversion, psychoticism and neuroticism are major personality traits. These broad traits can then be sub-categorised into a series of minor personality traits. Hans and Sybil developed the Eysenck Personality Questionnaire, with which they assessed the traits used in the PEN model.