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

Powers, Simon T.; Lehmann, Laurent


Laurent Lehmann


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 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.

Journal Article Type Article
Acceptance Date Feb 11, 2016
Online Publication Date Mar 17, 2016
Publication Date 2017-05
Deposit Date Jun 3, 2016
Publicly Available Date Mar 18, 2017
Print ISSN 1464-7931
Electronic ISSN 1469-185X
Publisher Wiley
Peer Reviewed Peer Reviewed
Volume 92
Issue 2
Pages 902-920
Keywords Sociality; cooperation; altruism; reciprocity; punishment; relatedness; cultural evolution; group size; diminishing returns; institutions;
Public URL
Publisher URL
Contract Date Jun 3, 2016


When is bigger better? The effects of group size on the evolution of helping behaviours (1.2 Mb)

Copyright Statement
This is the peer reviewed version of the following article: Powers, S. T. and Lehmann, L. (2017), When is bigger better? The effects of group size on the evolution of helping behaviours. Biol Rev, 92: 902–920. doi:10.1111/brv.12260, which has been published in final form at This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

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