Section S1: We explicitely derive the distributions of group size that both types expect to encounter in the case of group formation by differential attachment.

Figure S1: Theoretical group size distributions for social and asocial individuals.

Section S2: We formulate the condition for social behavior to be altruistic in the general formulation of section 2 and explicit this condition for group formation by differential attachment.

Section S3: We describe in details the algorithm used to validate by simulation the model of section 3 in the manuscript.

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