Eysenck proposed a personality theory that classified people into two types: introverts who are shy and keep thoughts/emotions private, and extroverts who are outgoing and expressive. He suggested four quadrants based on introversion/extroversion and stability: A) introverted and unstable, B) extroverted and unstable, C) introverted and stable, or D) extroverted and stable, with B being most prone to deviant behavior due to low social control.