Abstract
Recent years have seen great interest in the suggestion that between-group aggression and within-group altruism have coevolved. However, these efforts have neglected the possibility that warfare - via its impact on demography - might influence human social behaviours more widely, not just those directly connected to success in war. Moreover, the potential for sex differences in the demography of warfare to translate into sex differences in social behaviour more generally has remained unexplored. Here, we develop a kin-selection model of altruism performed by men and women for the benefit of their groupmates in a population experiencing intergroup conflict. We find that warfare can promote altruistic, helping behaviours as the additional reproductive opportunities winners obtain in defeated groups decrease harmful competition between kin. Furthermore, we find that sex can be a crucial modulator of altruism, with there being a tendency for the sex that competes more intensely with relatives to behave more altruistically and for the sex that competes more intensely with non-relatives in defeated groups to receive more altruism. In addition, there is also a tendency for the less-dispersing sex to both give and receive more altruism. We discuss implications for our understanding of observed sex differences in cooperation in human societies.
Highlights
The last decade has seen considerable multidisciplinary interest in understanding the potential evolutionary links between warfare and the high levels of altruism observed in human societies (Bauer et al 2016; Rusch 2014; Rusch et al 2016; Turchin 2015)
Patterns of relatedness and competition, in turn, are known to modulate incentives to perform social behaviours (Frank 1998) and, in this way, warfare demographies could influence other forms of within-group altruism – not just those that result in greater success in battle and that have been the focus of mathematical analyses so far ( Bowles 2006, 2009; Choi and Bowles 2007; Garcia and van der Bergh 2011; Lehmann and Feldman 2008; Micheletti et al 2017, 2018)
We first investigate which sex is favoured to perform more altruism, focusing on altruistic behaviours that are uniformly beneficial for the group, that is they are not targeted towards a specific sex of groupmates
Summary
The last decade has seen considerable multidisciplinary interest in understanding the potential evolutionary links between warfare and the high levels of altruism observed in human societies (Bauer et al 2016; Rusch 2014; Rusch et al 2016; Turchin 2015). Patterns of relatedness and competition, in turn, are known to modulate incentives to perform social behaviours (Frank 1998) and, in this way, warfare demographies could influence other forms of within-group altruism – not just those that result in greater success in battle and that have been the focus of mathematical analyses so far ( Bowles 2006, 2009; Choi and Bowles 2007; Garcia and van der Bergh 2011; Lehmann and Feldman 2008; Micheletti et al 2017, 2018). Since the evolutionary interests of men and women often diverge and the two sexes play different roles in social groups (Low 2015), elucidating the possible demographic drivers of sexspecific altruism is key to illuminate human sociality
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