Philosophical Transactions in Genetics 1 Philos. Trans. Genet. 2: 1-16 (2013) A family study of reactive and proactive aggression in a South African population Laubscher, N., Odendaal, Z., Schneider, S. & Spies, J.J. Department of Genetics (116), University of the Free State, P.O. Box 339, Bloemfontein 9300 Abstract The objective of this study was to determine upper-limit heritability estimates, as well as the influence of the non-shared environment on reactive- and proactive aggression in a sample of South African families. In addition the possible influence of assortative mating was determined. The sample consisted of 88 participants from 23 nuclear families. From the 88 participants, 79 parent-offspring pairs were obtained. Participants completed an aggression questionnaire. Upper-limit heritability estimates were determined by means of two different statistics. Firstly, upper-limit heritability was calculated by single parent-offspring correlation coefficients, and then doubling these coefficients. Secondly, upper-limit heritability was determined by calculating midparent-offspring regression coefficients. Latter named statistic controls for assortative mating, and by comparing the results of the two types of statistics, the possible role of assortative mating was determined. The findings showed that 41.2% to 42% of the variance in reactive aggression in this sample can be attributed to the joined effects of additive genetic and non-shared environmental influences (upper-limit heritability). For proactive aggression, the percentages dropped to between 6% and 12.9%. Conversely, the influence of the non-shared environment accounted for 58% to 58.8% of the variance in reactive aggression, and for 87.1% to 94% of the variance in proactive aggression. Assortative mating played no role in this sample. In conclusion, the influence of genetic and shared environmental factors on reactive aggression seemed much more profound than on proactive aggression. The non-shared environment is important in influencing aggressive behaviour. Keywords: Non-shared environment, Proactive aggression,