Abstract
Aggressive interactions between groups are frequent in human societies and can bear significant fitness costs and benefits (e.g. death or access to resources). During between-group competitive interactions, more cohesive groups (i.e. groups formed by individuals who cooperate in group defence) should out-perform less cohesive groups, other factors being equal (e.g. group size). The cost/benefit of between-group competition are thought to have driven correlated evolution of traits that favour between-group aggression and within-group cooperation (e.g. parochial altruism). Our aim was to analyse whether the proximate relationship between between-group competition and within-group cooperation is found in 3–10 years old children and the developmental trajectory of such a relationship. We used a large cohort of children (n = 120) and tested whether simulated between-group competition increased within-group cooperation (i.e. how much of a resource children were giving to their group companions) in two experiments. We found greater within-group cooperation when groups of four children were competing with other groups then in the control condition (no between-group competition). Within-group cooperation increased with age. Our study suggests that parochial altruism and in-group/out-group biases emerge early during the course of human development.
Highlights
Individuals in groups frequently face collective action problems involving, for example, paying taxes, taking part in strike actions or in a conflict against an opposing country or group[1,2,3]
In order to rule out the possibility that our results were biased by the different comprehension of the experiments by younger and older children, we first tested our predictions on the whole cohort of children and repeated the analyses, for our two test variables, restricted to children who were ≥six years old
The results of our two experiments support our first hypothesis that there is a positive relationship between between-group competition (BGC) and within-group cooperation
Summary
Individuals in groups frequently face collective action problems involving, for example, paying taxes, taking part in strike actions or in a conflict against an opposing country or group[1,2,3]. According to this hypothesis[17,20], the high risk of warfare-associated costs for individual fitness has driven the evolution of a series of cognitive and behavioural traits that favour parochial altruism (i.e. the propensity to be prosocial towards members of own group and aggressive towards out-group) and the recognition of in-group membership, which decrease the occurrence of free-riding when there is an out-group threat Supporting this view, in-group/out-group biases, fear of out-group, and social norms on fairness and punishment of free-riders have been observed in virtually every human society[30,31,32,33,34]
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