Dimensions of Personality

Eysenck identified neuroticism (which he called the 'P' factor) and extraversion–introversion as the two principal dimensions of personality . He derived them through factor analysis of correlations among thirty-nine items from psychiatric records of 700 servicemen drawn from a larger sample of 7,300 patients at Mill Hill Emergency Hospital . The first factor was a general factor of neuroticism, whose opposite pole corresponded to traits like perseverance, conscientiousness, and strength of will . The second factor covered the familiar introversion–extraversion dichotomy. Both factors emerged as statistically independent, meaning they described distinct aspects of personality rather than overlapping ones .

The book was the product of an unusually large collaborative effort. According to the publisher's description, more than three dozen separate researches were carried out on some 10,000 normal and neurotic subjects by a research team of psychologists and psychiatrists . The core findings were based on intensive analysis of psychiatric records from 7,300 male and female service patients, mostly neurotic, admitted to Mill Hill Emergency Hospital during the war . Eysenck emphasized the close collaboration between psychologists and psychiatrists as a special feature of the work, arguing that both disciplines benefit from working together on their shared problems .

The book challenged two dominant assumptions. First, it rejected the view that personality traits were primarily determined by environmental influences—an assumption that derived in part from the excessively environmentalist behaviorist approach prevalent at the time . Second, it questioned the reliance on numerous primary factors and oblique factor solutions favored by American researchers like Cattell. Eysenck advocated focusing on large, second-order factors, arguing that extraversion and neuroticism are the most important and readily replicable dimensions . His contention that these two factors were fundamental has largely stood the test of time .

Eysenck believed that the exploration of personality would have reached an advanced state much earlier had such collaboration been the rule rather than the exception . The book's data came primarily from psychiatric records, but its methods were psychological—factor analysis, experimental measurement, and operational definitions. A contemporary review in The Eugenics Review described the book as 'an application of psychological methods, some of them ingenious and original, to psychiatric material,' bringing closer together two domains whose relation was analogous to that between physiology and pathology . This interdisciplinary approach was central to Eysenck's ambition of establishing psychology as a natural science.

The book's publisher notes that it demonstrated 'the possibility of testing personality theory experimentally' . By defining personality dimensions operationally through strictly quantitative procedures, Eysenck showed that constructs like neuroticism and extraversion could be measured and correlated with external criteria rather than remaining speculative concepts. The book included an appendix on 'An Experimental Study in the Methodology of Factor Analysis' and another on 'Static Ataxia as an Index of Neuroticism' . This empirical orientation distinguished Eysenck's approach from psychoanalytic and purely descriptive personality theories that lacked testable hypotheses.

Dimensions of Personality is the descriptive and taxonomic foundation; The Biological Basis of Personality (1967) is the causal and physiological extension. The 1947 book identified the dimensions but did not specify their biological substrates. Eysenck later proposed that extraversion–introversion was caused by variability in cortical arousal mediated by the ascending reticular activating system, and that neuroticism related to limbic system activation . A 2016 research evaluation described Eysenck's ambition as building a theory with antecedents in DNA, advancing through biological intermediaries to the dimensional analysis of behavior—a program that began with the 1947 volume .