Eysenck

Hans Jürgen Eysenck (1916–1997) was a British psychologist of German origin who became one of the most influential figures in personality theory and psychometrics. He is best known for developing the PEN model of personality and for co-developing the Eysenck Personality Questionnaire (EPQ). His work emphasized a biological, largely genetic basis for individual differences in temperament and behavior, and he argued that personality dimensions could be traced to brain physiology and arousal.

The PEN model posits three dimensions: Psychoticism, Extraversion, and Neuroticism. Psychoticism relates to traits such as aggressiveness, impersonal attitudes, and tough-mindedness; Extraversion to sociability and activity; Neuroticism to emotional instability and vulnerability to stress. Eysenck proposed that extraversion and neuroticism reflect underlying cortical arousal and autonomic nervous system responses, shaping how people seek stimulation and cope with stress. The EPQ and its variants (such as the EPQ-R and the EPI) are standardized questionnaires designed to measure these dimensions and are used in clinical, educational, and organizational contexts.

Beyond personality, Eysenck conducted research on intelligence, psychopathology, and criminology, consistently arguing for a substantial biological component to individual differences. His work influenced debates about nature versus nurture and the assessment of personality in both research and applied settings. In the later years of his career and posthumously, some of his research faced scrutiny regarding data integrity and methodological practices, generating ongoing discussion within the field.

Eysenck spent most of his career at the Institute of Psychiatry in London and published extensively, shaping modern trait theory and popularizing biologically informed approaches to psychology. His influence persists in the continued use of EPQ instruments and in debates about the biological basis of personality.