Eysenck is especially well-known for his classification of personality-types. He divided people into four broad types: Extrovert-Stable, Extrovert-Unstable, Introvert-Stable and Introvert-Unstable. These types are fairly self-apparent in our daily lives, so no surprises there. However, he found that certain types are drawn to particular subjects of study, works of art and political affiliations in markedly predictable ways - and that is where things become really interesting.
Extrovert-Unstable people are, according to Eysenck, more prone to insanity, extremism and authoritarianism than other personality types. Again, this is pretty much what we would expect. However, Eysenck soon found that his intellectual opponents in British universities were typically sociology students, many of whom resorted to threats of violence against him. Further research confirmed his hunch that students drawn to sociology were overwhelmingly Unstable-Extroverts, very seldom Stable-Introverts.