The inheritance of extraversion-introversion

Abstract

In this study an effort has been made to provide evidence for the existence of a factor of extraversion-introversion among children, similar to that found among adults, and to measure this factor. By and large, this attempt has been successful and the factorial analysis reported in this paper gives clear evidence of a strong factor of extraversion-introversion. It was hypothesized that the concept of extraversion-introversion, as operationally defined in the writer's previous work, would be closely parallel to Rorschach's concept of extratensive-introvertive personality. The inclusion of a number of R scores in the factor analysis made it possible to test this hypothesis, and the results on the whole favoured acceptance of this theory. Two further factors were isolated in the analysis, namely, one of intelligence and one of autonomic activity. These additional factors were found to be independent of each other and also to be independent of extraversion-introversion. Factor scores were calculated for all three factors for the members of the experimental populations. As the major aim of the investigation was to study the effects of heredity on extraversion-introversion, the subjects of the investigation were 13 pairs of male identical twins, 13 pairs of female identical twins, 13 pairs of male fraternal twins and 13 pairs of female fraternal twins. By using standard methods of intra-class correlation for different types of twins, it was shown that all three factors, identical twins resembled each other significantly more closely than did fraternal twins. This was regarded as proof that heredity played an important part in the determination of intelligence, extraversion, and autonomic reactivity.