Abstract

Interest in the evolutionary origins and drivers of warfare in ancient and contemporary small-scale human societies has greatly increased in the last decade, and has been particularly spurred by exciting archaeological discoveries that suggest our ancestors led more violent lives than previously documented. However, the striking observation that warfare is an almost-exclusively male activity remains unexplained. Three general hypotheses have been proposed, concerning greater male effectiveness in warfare, lower male costs, and patrilocality. But while each of these factors might explain why warfare is more common in men, they do not convincingly explain why women almost never participate. Here, we develop a mathematical model to formally assess these hypotheses. Surprisingly, we find that exclusively male warfare may evolve even in the absence of any such sex differences, though sex biases in these parameters can make this evolutionary outcome more likely. The qualitative observation that participation in warfare is almost exclusive to one sex is ultimately explained by the fundamentally sex-specific nature of Darwinian competition—in fitness terms, men compete with men and women with women. These results reveal a potentially key role for ancestral conditions in shaping our species' patterns of sexual division of labour and violence-related adaptations and behavioural disorders.

Highlights

  • We find that natural selection—including both direct and indirect effects [50 –57]—favours an increase in participation in warfare by an individual of sex i when

  • We developed and analysed a model of the co-evolution of male and female participation in warfare

  • Our results suggest an entirely novel explanation for why women do not participate in warfare

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Summary

Introduction

Recent contributions from multiple disciplines—including archaeology, psychology, evolutionary biology, and anthropology—have greatly deepened our understanding of warfare, which may be broadly defined as coalitionary intergroup aggression [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28]. In the vast majority of historical and contemporary hunter –gatherer and small-scale societies, women have only rarely participated in warfare in a direct way—i.e. in fighting—and their usual role, if any, has been a supporting one [4,9,17,29,31,32,33,34,35] This strong sex difference is observed in chimpanzees, which are our closest living relatives and are understood to be the only other primates that routinely engage in lethal intergroup conflict [11,36,37]. The puzzle is not why male participation in warfare is more common than female participation (we outline potential explanations for this directly below), but why this imbalance is commonly so extreme, i.e. women taking no part at all

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