Full context:   In the 1950s, British psychologist Hans Eysenck theorized that the trait of introversion-extraversion could be explained in terms of Clark Hull's drive theory of motivation. He later developed his own arousal theory to explain individual differences in the trait, suggesting that the brains of extraverts were chronically under-aroused, leading them to seek out stimulation from the environment. Introverts, being more cortically aroused, avoid overstimulating environments. The trait of introversion-extraversion would become one of three central traits in Eysenck's PEN theory of personality.