When is bigger better? The effects of group size on the evolution of helping behaviours

Abstract

Understanding the evolution of sociality in humans and other species requires understanding how selection on social behaviour varies with group size. However, the effects of group size are frequently obscured in the theoretical literature, which often makes assumptions that are at odds with empirical findings. In particular, mechanisms are suggested as supporting large-scale cooperation when they would in fact rapidly become ineffective with increasing group size. Here we review the literature on the evolution of helping behaviours (cooperation and altruism), and frame it using a simple synthetic model that allows us to delineate how the three main components of the selection pressure on helping must vary with increasing group size. The first component is the marginal benefit of helping to group members, which determines both direct fitness benefits to the actor and indirect fitness benefits to recipients. While this is often assumed to be independent of group size, marginal benefits are in practice likely to be maximal at intermediate group sizes for many types of collective action problems, and will eventually become very small in large groups due to the law of decreasing marginal returns. The second component is the response of social partners on the past play of an actor, which underlies conditional behaviour under repeated social interactions. We argue that under realistic conditions on the transmission of information in a population, this response on past play decreases rapidly with increasing group size so that reciprocity alone (whether direct, indirect, or generalised) cannot sustain cooperation in very large groups. The final component is the relatedness between actor and recipient, which, according to the rules of inheritance, again decreases rapidly with increasing group size. These results explain why helping behaviours in very large social groups are limited to cases where the number of reproducing individuals is small, as in social insects, or where there are social institutions that can promote (possibly through sanctioning) large-scale cooperation, as in human societies. Finally, we discuss how individually devised institutions can foster the transition from small-scale to large-scale cooperative groups in human evolution.

Cooperation can promote rescue or lead to evolutionary suicide during environmental change

“…2012; Shen et al. 2013; Powers and Lehmann 2017; Peña and Nöldeke 2018), and it matches at least some empirical observations: for example, the amount of time that meerkats spend on guard decreases with group size (Clutton‐Brock et al. 1999), and Pseudomonas “cheaters” that do not produce siderophores perform better at high cell densities (Ross‐Gillespie et al.…”

Changes in group size during resource shifts reveal drivers of sociality across the tree of life

“…In practice, measuring the selection pressures driving sociality is difficult. It may require performing one or more experiments for each potential benefit of sociality, and the magnitude of a benefit may vary nonlinearly with group size and across different environmental contexts 22, 25, 27, 29, 40–42 . As a result, it is likely infeasible to comprehensively measure all of the selection pressures governing sociality for any species.…”

Group size effects in social evolution

“…More generally, however, whether or not larger groups are less conducive to cooperation might depend on specific assumptions about group interactions. In particular, instances of positive group size effects have also been reported in the empirical literature (Isaac et al, 1994;Yip et al, 2008;Powers and Lehmann, 2017) and are of significant theoretical interest (Dugatkin, 1990;Shen et al, 2014;Powers and Lehmann, 2017;Cheikbossian and Fayat, 2018).…”

“…In this light, the analysis conducted here is also relevant to investigate group size effects in genetically structured populations, provided that the likely dependence of the inclusive gains from switching on group size is taken into account. Investigating the effects of group size on the evolution of cooperative behaviors under nontrivial population structure with the tools developed here would complement recent efforts in this area (Shen et al, 2014;Powers and Lehmann, 2017;Van Cleve, 2017). from which Eq.…”