In psychology, personality type refers to the psychological classification of individuals. In contrast to personality traits, the existence of personality types remains extremely controversial.[1][2] Types are sometimes said to involve qualitative differences between people, whereas traits might be construed as quantitative differences.[3] According to type theories, for example, introverts and extraverts are two fundamentally different categories of people. According to trait theories, introversion and extraversion are part of a continuous dimension, with many people in the middle.