Hans and Sybil Eysenck developed a widely recognized theory of personality, which emphasizes the role of temperament and genetically based differences in shaping individual traits. Their theory posits that biological factors primarily determine personality and can be understood through two main dimensions: extroversion/introversion and neuroticism/stability.

In the extroversion/introversion dimension, highly extroverted people are sociable, outgoing, and easily connect with others. In contrast, those high in introversion have a greater need for solitude, engage in solitary activities, and limit their social interactions. Regarding the neuroticism/stability dimension, individuals high in neuroticism often experience anxiety and possess an overactive sympathetic nervous system, causing them to enter a fight-or-flight response even under low-stress conditions. In contrast, people with high emotional stability require more stimulation to trigger their fight-or-flight response and are generally considered more emotionally resilient. Based on these two key dimensions — extroversion/introversion and neuroticism/stability — the Eysencks divided personality types into four quadrants. These quadrants align with the four temperaments described by the ancient Greeks. The melancholic type is characterized by introversion and high neuroticism, making individuals moody, anxious, and pessimistic. The choleric type is extroverted and high in neuroticism, leading to traits such as being touchy, restless, and aggressive. The phlegmatic type is introverted and emotionally stable, displaying passive, peaceful, and controlled qualities. Lastly, the sanguine type is extroverted and stable, characterized by sociability, liveliness, and a carefree nature.

The Eysencks later introduced a third dimension, psychoticism versus superego control. Individuals high in psychoticism tend to be impulsive, independent, and sometimes antisocial, while those with high superego control display greater empathy, cooperation, and impulse regulation. This addition expanded their theory, offering a more nuanced understanding of personality.

Eysenck's model provides a biological framework for examining personality, highlighting the interplay between genetic factors and psychological traits. It remains a foundational theory in personality psychology, contributing significantly to our understanding of individual differences.