Journal of Personality and Social Psychology1975, Vol. 32, No. 1, 102-112The Nature of Extraversion: A Genetical AnalysisLindon EavesDepartment of Genetics,University of Birmingham,Birmingham, EnglandHans EysenckInstitute of Psychiatry,University of London,London, EnglandA biometrical-genetical analysis of twin data to elucidate the determinants ofvariation in extraversion and its components, sociability and impulsiveness, revealedthat both genetical and environmental factors contributed to variation in extra-version, to the variation and covariation of its component scales, and to the inter-action between subjects and scales. A large environmental correlation between thescales suggested that environmental factors may predominate in determining theunitary nature of extraversion. The interaction between subjects and scales de-pended more on genetical factors, which suggests that the dual nature of extraver-sion has a strong genetical basis. A model assuming random mating, additive geneaction, and specific environmental effects adequately describes the observed vari-ation and covariation of sociability and impulsiveness. Possible evolutionary impli-cations are discussed.One of the central problems in personalityresearch has been the question of whether suchhigher order factors as extraversion can beregarded in any meaningful sense as unitary orwhether there are several independent factors,such as "sociability" and "impulsiveness,"which should not be thrown together arti-ficially. Carrigan (1960) concluded her surveyof the literature by saying that "the uni-dimensionality of extraversion/introversion hasnot been conclusively demonstrated" (p. 355);she further pointed out that several jointanalyses of the Guildford and Cattell question-naires show that at least two independentfactors are required to account for the inter-correlations between the extraversion-impul-siveness variables. These two factors, shesuggested, may correspond to the Europeanconception of extraversion, with its emphasis onimpulsiveness and weak superego controls, andthe American conception, with its emphasis onsociability and ease in interpersonal relations.Eysenck and Eysenck (1963) have reportedquite sizable correlations between sociabilityand impulsiveness, a conclusion replicated byWe are indebted to the Colonial Research Fund andthe British Medical Research Council for their supportof the investigation. We are grateful to J. Kasriel forthe collection of data and to the twins for their con-tinued cooperation.Requests for reprints should be sent to Lindon Eaves,Department of Genetics, University of Birmingham,Birmingham, B15 2TT, England.Sparrow and Ross (1964); this would suggestthat there is a close connection between thetwo conceptions (Eysenck & Eysenck, 1969).Furthermore, Eysenck and Eysenck (1967)have shown that the correlations of extraversionitems (whether sociability or impulsiveness)with subjects' reactions on a physiological testdevised on theoretical grounds were propor-tional to their loadings on the extraversionfactor. The recognition that extraversion is aunitary factor in behavior is thus vindicatedby prediction from a psychological theory asmuch as by a correlation between primaryfactors (Eysenck, 1967).We now develop a model for the geneticaland environmental determinants of extraver-sion and of its primary components, sociabilityand impulsiveness. Our intention is to analyzethe phenotypic variation and covariation ofsociability and impulsiveness into their geneti-cal and environmental components in order todetermine, as far as our data permit: (a) thesimplest model for the genetical and environ-mental variation of extraversion considered asa unitary trait and (b) the simplest model forthe genetical and environmental determinationof the interaction between subjects and thecomponent tests of extraversion, sociability andimpulsiveness.In fulfilling these aims, we are led to com-pare the unitary and dual models of extra-version with regard to their relative contri-102THE NATURE OF EXTRAVERSION103butions to the representation of both geno-typic and environmental determinants ofvariation among the responses of subjects to apersonality inventory.Earlier research from the standpoint of thepsychological theory underlying this work hasmainly been concerned with the analysis ofextraversion as a unitary trait (Shields, 1962).Claridge, Canter and Hume (1973) reportedanalyses of extraversion, sociability, and impul-siveness, but these authors themselves admittedthat their samples were too small to justifythe kind of analysis we attempt here. Ourmodel will be derived from an analysis of twindata and will, therefore, inevitably reflect thelimitations of twin studies as sources of geneti-cal information (Jinks & Fulker, 1970). Eventwin studies, however, have seldom been usedto best advantage. We hope that our particularanalysis will have the additional virtue ofdemonstrating how twin data in general maybe manipulated to test simple hypothesesabout the causes of variation. We have adoptedthe methods and notation of biometrical gene-tics (Mather & Jinks, 1971) because webelieve them to be the most precise and general,while embodying a denned procedure for theanalysis of continuous variation which may beextended readily to the analysis of humanbehavior (Jinks and Fulker, 1970).DATAThe analysis is based on the responses of837 pairs of adult volunteer twins to an 80-itempersonality inventory. Of these items, 13 formeda scale of sociability, and 9 items were scoredto provide a measure of impulsiveness. Therelevant items are given in Table 1. On thebasis of a short questionnaire concerningsimilarity during childhood, the twins wereclassified as monozygotic or dizygotic.1 Such aprocedure is surprisingly reliable (Cederlb'f,Friberg, Jonsonn, & Kaij, 1961). A sample of1 The twins were asked: (a) "Do you differ markedlyin physical appearance and coloring?" and (b) "Inchildhood were you frequently mistaken by people whoknew you?" If consistent replies were not given,reference was made to previous questionnaires, twins'letters, and additional information in an attempt toassess zygosity. Many of the twins have been blood-typed subsequently, and the original diagnoses havegenerally been confirmed (Kasriel, J., personal communi-cation, December 1974).TABLE 1PERSONALITY INVENTORY ITEMS INCLUDEDIN THE ANALYSISItem18. Do you suddenly feel shy when you wantto talk to an attractive stranger?23. Generally, do you prefer reading