Many important variables in behavioral development are presumed to be unrelated because of repeated failures to obtain substantial correlations. In this article, we explore the possibility that such null findings have often been due to failures to aggregate. The principle of aggregation states that the sum of a set of multiple measurements is a more stable and representative estimator than any single measurement. This greater representation occurs because there is inevitably some error associated with measurement. By combining numerous exemplars, such errors of measurement are averaged out, leaving a clearer view of underlying relationships. We illustrate the usefulness of this principle in 12 major areas of developmental research in which the issue of negligible correlations figures prominently: (a) the validity of judges' ratings, (b) the cross-situational consistency of moral character and personality, (c) the longitudinal stability of personality, (d) the coherence of stages of cognitive development, (e) metacognition, (f) the attitude-behavior relationship, (g) the personality-behavior relationship, (h) the role-taking/altruism relationship, (i) the moral-judgment/altruism relationship, (j) the legitimacy of the construct of attachment, (k) the existence of sex differences, and (1) the assessment of emotionality in animals. In a final section, we also discuss the implications of the principle of aggregation for conducting experimental research. A preliminary version of this paper was presented at the Merrill-Palmer Society, Detroit, May 1982. We would like to thank Jack Block, John Borkowski, Douglas Jackson, and Linda Siegel for discussions of the issues involved in this paper. Requests for reprints should be sent to J.

The culture-only (0% genetic-100% environmental) and the hereditarian (50% genetic-50% environmental) models of the causes of mean Black-White differences in cognitive ability are compared and contrasted across 10 categories of evidence: the worldwide distribution of test scores, g factor of mental ability, heritability, brain size and cognitive ability, transracial adoption, racial admixture, regression, related life-history traits, human origins research, and hypothesized environmental variables. The new evidence reviewed here points to some genetic component in Black-White differences in mean IQ. The implication for public policy is that the discrimination model (i.e., Black-White differences in socially valued outcomes will be equal barring discrimination) must be tempered by a distributional model (i.e., Black-White outcomes reflect underlying group characteristics). Section 1: BackgroundThroughout the history of psychology, no question has been so persistent or so resistant to resolution as that of the relative roles of nature and nurture in causing individual and group differences in cognitive ability (Degler, 1991;Loehlin, Lindzey, & Spuhler, 1975). The scientific debate goes back to the mid-19th century (e.g., Galton, 1869;Nott & Glidden, 1854). Starting with the widespread use of standardized mental tests in World War I, average ethnic and racial group differences were found. Especially vexing has been the cause(s) of the 15-point Black-White IQ difference in the United States.In 1969, the Harvard Educational Review published Arthur Jensen's lengthy article, "How Much Can We Boost IQ and School Achievement?" Jensen concluded that (a) IQ tests measure socially relevant general ability; (b) individual differences in IQ have a high heritability, at least for the White populations of the United States and Europe; (c) compensatory educational programs have proved generally ineffective in raising the IQs or school achievement of individuals or groups; (d) because social mobility is linked to ability, social class differences in IQ probably have an appreciable genetic component; and tentatively, but most controversially, (e) the mean Black-White group difference in IQ probably has some genetic component. Jensen's (1969) article was covered in Time, Newsweek, Life, U.S. News & World Report, and New York Times Magazine. His conclusions, the theoretical issues they raised, and the public policy recommendations that many saw as stemming directly from them were dubbed "Jensenism," a term which entered the J. Philippe Rushton, Department of Psychology, The University of Western Ontario, London, Ontario, Canada; Arthur R. Jensen, School of Education, University of California, Berkeley.Correspondence concerning this article should be addressed to J. Philippe Rushton, Department of Psychology, The University of Western Ontario, London, Ontario N6A 5C2, Canada. E-mail: rushton@uwo.ca Psychology, Public Policy, and Law 2005, Vol. 11, No. 2, 235-294 Copyright 2005 by the American Psychological Association 1076-8971/0...

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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