Personality theorists Hans and Sybil Eysenck developed a theory centered on temperament, which they defined as inborn, genetically based personality differences. Believing that personality is largely governed by biology, they identified two primary dimensions: extroversion versus introversion and neuroticism versus stability. Their model suggests that extroverts are sociable and outgoing, while introverts prefer solitude. Similarly, individuals high in neuroticism are anxious and have a reactive stress response, whereas those high in stability are more emotionally resilient. A third dimension, psychoticism versus superego control, was later added to their model.