All the evidence to date suggests the ... overwhelming importance of genetic factors in producing the great variety of intellectual differences which we observe in our culture, and much of the difference observed between certain racial groups.— HJ Eysenck, Race, Intelligence and Education, 1971, London: Temple Smith, p. 130
In 1951, Eysenck's first empirical study into the genetics of personality was published. It was an investigation carried out with his student and associate Donald Prell, from 1948 to 1951, in which identical (monozygotic) and fraternal (dizygotic) twins, ages 11 and 12, were tested for neuroticism. It is described in detail in an article published in the Journal of Mental Science. Eysenck and Prell concluded that, "The factor of neuroticism is not a statistical artifact, but constitutes a biological unit which is inherited as a whole....neurotic predisposition is to a large extent hereditarily determined."
Eysenck was Professor of Psychology at the Institute of Psychiatry, King's College London, from 1955 to 1983. He was a major contributor to the modern scientific theory of personality and a brilliant Teacher who helped found treatment for mental illnesses. Eysenck also created and developed a distinctive dimensional model of personality structure based on empirical factor-analytic research, attempting to anchor these factors in biogenetic variation. In 1981, Eysenck became a founding member of the World Cultural Council. He was the founding Editor of the international journal Personality and Individual Differences, and wrote about 80 books and more than 1600 journal articles. His son Michael Eysenck is also a noted psychology professor. Hans Eysenck died of a brain tumour in a London hospice in 1997. He was an atheist.
The two personality dimensions extraversion and neuroticism were described in his 1967 book Dimensions of Personality. It is Common practice in personality psychology to refer to the dimensions by the first letters, E and N.
The third dimension, psychoticism, was added to the model in the late 1970s, based upon collaborations between Eysenck and his wife, Sybil B. G. Eysenck (e.g., Eysenck & Eysenck, 1976), who is the current Editor of Personality and Individual Differences.
Barnett quotes additional criticism of Race, Intelligence and Education from Sandra Scarr-Salapatek, who wrote in 1976 that Eysenck's book was "generally inflammatory" and that there "is something in this book to insult almost everyone except WASPs and Jews." Scarr was equally critical of Eysenck's hypotheses, one of which was the supposition that slavery on plantations had selected African Americans as a less intelligent sub-sample of Africans. Scarr also criticised another statement of Eysenck on the alleged significantly lower IQs of Italian, Spanish, Portuguese and Greek immigrants in the US relative to the populations in their country of origin. "Although Eysenck is careful to say that these are not established facts (because no IQ tests were given to the immigrants or nonimmigrants in question?" Scarr writes that the careful reader would conclude that "Eysenck admits that scientific evidence to date does not permit a clear choice of the genetic-differences interpretation of black inferiority on intelligence tests," whereas a "quick reading of the book, however, is sure to leave the reader believing that scientific evidence today strongly supports the conclusion that US blacks are genetically inferior to whites in IQ." Some of Eysenck's later work was funded from the Pioneer Fund, an organization often criticised for allegedly promoting scientific racism, However, Eysenck himself was vehemently opposed to racism. As Eysenck stated, "My hatred of Hitler and the Nazis, and all they stood for, was so overwhelming that no argument could counter it (p. 40)."
Eysenck was accused of being a supporter of political causes on the extreme right. Connecting arguments were that Eysenck had articles published in the German newspaper National Zeitung, which called him contributor, and in Nation und Europa, and that he wrote the preface to a book by a far-right French Writer named Pierre Krebs, Das unvergängliche Erbe, that was published by Krebs' Thule Seminar. Linguist Siegfried Jäger interpreted the preface to Krebs' book as having, "...railed against the equality of people, presenting it as an untenable ideological doctrine." In the National Zeitung Eysenck reproached Sigmund Freud for alleged trickiness and lack of frankness. Other incidents that fuelled Eysenck's critics like Michael Billig and Steven Rose include the appearance of Eysenck's books on UK National Front's list of recommended readings and an interview with Eysenck published by National Front's Beacon (1977) and later republished in the US neo-fascist Steppingstones; a similar interview had been published a year before by Neue Anthropologie, described by Eysenck's biographer Roderick Buchanan as a "sister publication to Mankind Quarterly, having similar contributors and sometimes sharing the same articles." Eysenck also wrote an introduction for Roger Pearson's Race, Intelligence and Bias in Academe. In this introduction to Pearson's book, Eysenck retorts that his critics are "the scattered troops" of the New Left, who have adopted the "psychology of the fascists". Eysenck book The Inequality of Man, translated in French as L'Inegalite de l'homme, was published by GRECE's publishing house, Éditions Corpernic. In 1974 Eysenck became a member of the academic advisory council of the Mankind Quarterly, joining those associated with the journal in attempting to reinvent it as a more mainstream academic vehicle. Billig asserts that in the same year Eysenck also became a member of the comité de patronage of GRECE's Nouvelle École.
In 1994, he was one of 52 signatories on "Mainstream Science on Intelligence", an editorial written by Linda Gottfredson and published in the Wall Street Journal, which declared the consensus of the signing scholars on issues related to intelligence research following the publication of the book The Bell Curve. Gottfredson described the drafting of the statements on intelligence and process of gathering signatures on that document in a 1997 editorial in the journal Intelligence. Eysenck includes the entire text of the 1994 editorial (including the lead paragraphs mentioning The Bell Curve and twenty-five propositions about human intelligence) in his 1998 book Intelligence: A New Look, saying, "I did not find any particular discrepancies between my account" and the statements in that editorial.
Jeffrey Alan Gray, a former student of Eysenck's, developed a comprehensive alternative theoretical interpretation (called Gray's biopsychological theory of personality) of the biological and psychological data studied by Eysenck – leaning more heavily on animal and learning Models. Currently, the most widely used model of personality is the Big Five model. The purported traits in the Big Five model are as follows:
Eysenck’s attitude was summarised in his autobiography Rebel with a Cause: "I always felt that a scientist owes the world only one thing, and that is the truth as he sees it. If the truth contradicts deeply held beliefs, that is too bad. Tact and diplomacy are fine in international relations, in politics, perhaps even in business; in science only one thing matters, and that is the facts." He was one of the signers of the Humanist Manifesto.