Evolutionary theory of human warfare: genes, individuals, groups

Abstract

Recent years have seen an explosion of multidisciplinary interest in coalitionary intergroup aggression – i.e. warfare – in human societies, and considerable advances in our understanding of its origins and evolutionary-ecological drivers. However, the study of human warfare has largely neglected the possibility that different parties – e.g. men versus women, younger versus older generations, or attacking versus defending groups – might have different incentives to influence the expression of warfare-related behaviours, which may result in conflicts of interest within and between groups and sex differences in behaviour. In this thesis, I develop a mathematical evolutionary framework, based on kin-selection theory, to investigate such differences in incentives, with special attention to sex-specific demography as a potential driver of conflicts of interest at multiple levels of the biological organisation. I find that: (a) the ecology of warfare can drive the evolution of sex-biased dispersal, which in turn modulates intrafamily and intragenomic conflicts over warfarerelated behaviours, with the latter leading to parent-of-origin-specific gene expression (genomic imprinting) and maladaptive behavioural disorders; (b) almost-exclusively male warfare can be driven by an evolutionary feedback between male and female participation in battle, rather than fundamental differences between the sexes; (c) sex is a fundamental modulator of altruism in the context of the demography of warfare, with the sex that competes more globally and/or is more philopatric being favoured to behave more altruistically towards same-sex groupmates than opposite-sex ones; (d) conflicts of interest within and between attacking and defending groups inhibit the formation of military alliances and the shift to large-scale human societies more generally. Taken together, these results suggest that differences in incentives for different parties in the context of warfare – often driven by sex-biases in demographic parameters – result in behavioural sex differences and conflicts of interest within and between different organisational levels.