Psychologists Hans and Sybil Eysenck proposed that biology significantly influences personality and identified two key personality dimensions: extroversion/introversion and neuroticism/stability. Extroverts are sociable and outgoing, while introverts prefer solitude. Highly neurotic individuals are anxious with an overactive stress response, whereas emotionally stable individuals require more stimulation to trigger stress. Then, the dimension psychoticism versus superego control was added. The theory categorizes people into four quadrants based on these two dimensions, which are often likened to the four Greek temperaments: melancholic, choleric, phlegmatic, and sanguine.