The Biological Basis of Personality

Eysenck argues that individual differences in extraversion–introversion are caused by variation in cortical arousal, mediated by the ascending reticular activating system (ARAS) in the brainstem . Introverts are theorized to have chronically higher resting levels of cortical arousal, making them easily overstimulated and leading them to avoid intense or novel situations. Extraverts have lower resting arousal and therefore seek external stimulation—social interaction, novelty, excitement—to achieve an optimal level of arousal . This biological postulate replaced Eysenck's earlier Pavlovian-inspired excitation–inhibition model with a more Western neurophysiological framework . The 1967 book represents his third theoretical phase, where he dug deeper to find biological causes underlying the psychological concepts of emotion, excitation, and inhibition .

The Biological Basis of Personality is Eysenck's third phase of theorizing, following his earlier descriptive and taxonomic work in Dimensions of Personality (1947) and The Scientific Study of Personality (1952) . The earlier books used factor analysis to identify the major dimensions—extraversion and neuroticism—but did not specify their biological causes. The 1967 volume shifts from description to causal explanation, attempting to ground the psychological constructs in identifiable neurological and physiological mechanisms. Sybil Eysenck notes in her preface that this book paved the way for a 'marriage' of the experimental and individual difference approaches in personality psychology . The dimensional framework survived this transition; the specific biological postulates did not fare as well.

Eysenck's drug postulate states that the effects of extraversion–introversion on conditioning should correspond to the effects of depressant and stimulant drugs . If introverts are chronically high in cortical arousal, then depressant drugs (like barbiturates) should shift their behavior toward extraverted patterns, while stimulant drugs (like caffeine or amphetamines) should shift extraverts toward introverted patterns. This postulate provided a testable prediction linking the personality dimension to its proposed neurological substrate. Eysenck's experiments on drugs and personality revealed that sedative and stimulating drugs reflected different levels of cortical arousal of the ARAS . The drug postulate was part of Eysenck's broader strategy of using pharmacological manipulations as experimental tests of his biological theory.

Eysenck applied the Yerkes–Dodson law to his arousal theory, proposing an inverted U-shape relationship between stimulus intensity and performance . Very weak and very intense stimuli both produce poor outcomes, while intermediate levels yield optimal performance. Because introverts and extraverts differ in baseline arousal, they reach their optimal performance at different levels of external stimulation. Introverts, already high in arousal, perform best under low-stimulation conditions and are easily overwhelmed by intense stimuli. Extraverts, with lower baseline arousal, require more intense stimulation to reach the same optimal point. This principle explained a wide range of empirical findings on personality and performance and became a central feature of Eysenck's arousal theory in the 1967 book and subsequent work.

The dimensional framework has proven remarkably durable. Extraversion and neuroticism are now standard factors in the Big Five model, and a 1997 Nature obituary noted that there is 'widespread agreement that his essential postulates were correct' regarding the existence of a few fundamental and independent personality dimensions . However, the specific biological details have not aged well. A 2016 review concluded that Eysenck's later revision of the psychoticism dimension was 'fatally flawed' and that the details of his theorizing had 'serious deficiencies' . A Taylor & Francis overview states that the biological aspects of Eysenck's theory 'seem out of date' . Jeffrey Gray's revised reinforcement sensitivity theory overtook Eysenck's formulation as the dominant psychobiological model .

Critics pointed to methodological problems in the research underlying the theory. Colleagues noted that Eysenck's samples were not representative—his claims about the British middle class were based on younger, better-educated respondents . Different political groups were recruited through different methods, and scores from groups of vastly different sizes were weighted equally . A 2016 review of the conditioning research noted 'operational and conceptual difficulties' with the conditionability measure itself, and observed that studies failing to support the theory 'tended to be dismissed by him or explained away' . These critiques do not invalidate the dimensional findings but undermine confidence in the specific neurological mechanisms Eysenck proposed.