Five questionnaires measuring altruistic and aggressive tendencies were completed by 573 adult twin pairs of both sexes from the University of London Institute of Psychiatry Volunteer Twin Register. The questionnaires measured altruism, empathy, nurturanoe, aggressiveness, and assertiveness. The intraclass correlations for the five scales, respectively, were .53, .54, .49, .40, and .52 for 296 monozygotic pairs, and .25, .20, .14, .04, and .20 for 179 same-sex dizygotic pairs, resulting in broad heritability estimates of 56%, 68%, 70%, 72%, and 64%. Additional analyses, using maximum-likelihood modelfitting, revealed approximately 50% of the variance on each scale to be associated with genetic effects, virtually 0% with the twins' common environment, and the remaining 50% with each twins' specific environment and/or error associated with the tea Correcting for the unreliability in the tests raised the maximum-likelihood heritabilities to approximately 60%. Age and sex differences vase also found: altruism increased over the age span from 19 to 60, whereas aggressiveness decreased, and, at each age, women had higher scores than men on altruism and lower snores on aggressiveness. Although psychological research on altruism and aggression has expanded over the last 20 years, the question of consistent patterns of individual differences has been much neglected. This article attempts to redress this situation. A related topic is concerned with the origins of personality traits. Although some schools of thought, including Freud, and the sociobiologists (Dawkins, 1976; Wilson, 1975), have stressed the importance of genetic and instinctive influences on human behavior: most current theorizing about human aggression and altruism emphasizes intraindividual variability acquired and modified through cognitive social learning (Bandura, 1977; Rushton, 1980). It is time that heritability estimates were brought into a discussion of these alternative viewpoints. Despite an unsympathetic zeitgeist, there is, in fact, a peat deal of evidence that personality traits (a) exist, (b) are longitudinally stable, (c) can be assessed by several converging indices, and (d) are inherited (Rushton, Russell, & Wells, 1985). The heritability of individual differences in behavior may be assessed by several methods (Plomin, DeFries, & MdZlearn" 1980). For example, selective breeding studies of animals may be undertaken, using crossfostering to control for upbringing. In humans, correlations may be calculated between scores on the trait in Much of the research in this article was carried out while the first
