ABSTRACT
EYSENCK has been particularly concerned with the question of the degree to which personality features are due to hereditary factors (Eysenck and Prell, 1951; Eysenck, 1956a). In the most recent study using items from the Eysenck questionnaires, Shields (1962) tested fraternal twins brought up together, identical twins brought up together and identical twins brought up in separation, and succeeded in showing not only that identical twins were much more alike with respect to extraversion and neuroticism than were fraternal twins, but also that identical twins brought up in separation were, if anything, more alike than were identical twins brought up together. On the basis of these and other studies Eysenck has come to the conclusion that a large proportion, possibly as much as three quarters of the total variance for differences between individuals with respect to extraversion and neuroticism, is due to hereditary factors. Support for this conclusion is found in the work of Lienert and Reisse (1961), Cattell et al. (1955), Gottesman (1963), Wilde (1964) and others. 1
