Parental investment and physical aggression among males

Abstract

Past research indicates that men are physically more aggressive than women. Socio-cultural explanations for this sex difference involve variants of learning theory and a tabula rasa psychology. Sexual selection theory provides a more coherent ultimate explanation for sex differences in this and other domains of behaviour. The key processes in sexual selection (preferential mate choice and intra-sexual competition) can be understood in terms parental investment theory. This suggests that the higher-investing sex (usually female) will tend to become a more limiting resource for the lower investing sex. In bi-parentally investing species (e.g. humans), male parental investment tends to be less than the whole but more than a half of the female investment (Trivers, 1972). This is because unlike males the variable portion of the female's investment potentially begins from a higher (non-zero) threshold. This suggests that there may be greater male than female variability in parental investment in bi-parentally investing species, and consequentially greater male variability in sexually selected attributes. In the first study the prediction of greater male variability was tested through meta-analyses of variance ratios for data sets involving sexually selected characteristics (including physical aggression) and those unlikely to have resulted from sexual selection (including anger and self-esteem). Variation was significantly greater for men than women for most of the former data sets including physical aggression), and was similar for men and women for the latter data sets, broadly supporting the predictions. This is consistent with the view that the magnitude of the sex difference in physical aggression may be a function of the proportion of low parentally investing males in the sample. A scale (the pilot Paternal Investment Questionnaire; PIQ) to measure male parental investment was designed and standardised in the second study: the PIQ showed moderately high internal reliability, but results for concurrent and construct validity testing were inconclusive, with some evidence for the latter (as predicted PIQ scores were negatively correlated with a measure of mating effort [infidelity intentions] and positively correlated with jealousy). The main contribution of this thesis is in highlighting the need to view sex differences in terms of the variability in parental investment both between and within the sexes.