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.