Espinosa

Received 2 July 2004 Accepted 16 September 2004 Published online 30 November 2004

Genetic and environmental contributions to pro-social attitudes: a twin study of social responsibility
J. Philippe Rushton
Department of Psychology, University of Western Ontario, London, Ontario N6A 5C2, Canada (rushton@uwo.ca) Although 51 twin and adoption studies have been performed on the geneticarchitecture of antisocial behaviour, only four previous studies have examined a genetic contribution to pro-social behaviour. Earlier work by the author with the University of London Institute of Psychiatry Adult Twin Register found that genes contributed approximately half of the variance to measures of self-report altruism, empathy, nurturance and aggression, including acts of violence. The present studyextends those results by using a 22-item Social Responsibility Questionnaire with 174 pairs of monozygotic twins and 148 pairs of dizygotic twins. Fortytwo per cent of the reliable variance was due to the twins’ genes, 23% to the twins’ common environment and the remainder to the twins’ non-shared environment. Keywords: altruism; social attitudes; twins; genes; cultural transmission

1.INTRODUCTION Ever since Darwin (1871), human beings have been viewed as a highly intelligent species that evolved ideologies of social responsibility and trust to make communal living more effective. Although people interpret ‘the general good’ in different ways, most feel some degree of social responsibility, despite wide-ranging individual differences. Twin, adoption and neurohormonal studies haveshown genetic variation in both pro-social and antisocial behaviour. Monozygotic (MZ) twin pairs share 100% of their genes whereas dizygotic (DZ) twin pairs share 50%, thus doubling the difference between their correlations provides one simple estimate of heritability (the proportion of total variance accounted for by genetic factors; Plomin et al. 2001). In an analysis of self-ratings from 850 pairsof high school MZ and DZ twins of both sexes, Loehlin & Nichols (1976) found 40% heritabilities for both males and females for clusters labelled argumentative, family quarrel and kind. Matthews et al. (1981) found 72% heritability for a selfreport adjective checklist measure of empathy of 114 MZ and 116 DZ middle-aged male twins. A study of 563 pairs of MZ and DZ twins by Rushton et al. (1986)found that 50% of the variance in altruism, empathy, nurturance and aggression was due to the genes with 50% due to environmental factors. An Altruism Scale asked respondents about the frequency with which they had engaged in specific acts such as ‘I have given directions to a stranger’ and ‘I have donated blood’. An Emotional Empathy Scale asked people to endorse positively keyed items such as ‘Ilike to watch people open presents’ and negatively keyed ones such as ‘I find it silly for people to cry out of happiness’. A Nurturance Scale asked for agreement with items such as ‘I like to take young people under my wing’, and an Aggressiveness Scale with items such as ‘some people think I have a violent temper’. Not only was 50% of the variance in altruism and in aggression attributable toadditive genetic influence in the study of Rushton et al. (1986), but very little, if any, of the variance was due to the twins’ common environment.
Proc. R. Soc. Lond. B (2004) 271, 2583–2585 doi:10.1098/rspb.2004.2941

Rather, the non-genetic variance was attributable to each twin’s non-shared environment. Interestingly, parents, peers and teachers see altruism and aggression as traits thatshould be socialized heavily. However, ca. 50% of the variance was associated with genetic influence and most of the 50% environmental variance appeared to be idiosyncratic to the particular twin. The pro-social and antisocial measures were negatively correlated (r ¼ À0:29). Consequently, Rushton (1996) asked the twins to report on acts of delinquency, including those of criminal violence. Had they...