Frank, S. A. 1996. Policing and group cohesion when resources vary. Animal Behaviour 52:1163-1169.

The transition from competing individuals to cooperative groups has occurred several times in evolutionary history. The puzzle is why selfish individuals did not subvert cohesive group behaviour by taking resources without contributing to the group's overall success. Kin selection and reciprocal altruism are the two standard explanations for group cohesion. But many groups have evolved into cooperative units when relatedness was low and opportunities were limited for the strategic alliances required for reciprocity. A new theory was recently proposed in which individuals invest some of their resources in repressing competition among group members. Such policing increases the fair distribution of resources in the group and enhances group cohesion. The surprising aspect of this theory is that low relatedness is more conducive to the spread of policing traits than is high relatedness. Here a new explanation is developed of the biological processes that favour policing. The model is then extended in two ways. First, more realism is added to the theory by accounting for the full range of costs and benefits associated with competitive and cooperative traits within groups. Second, another surprising result is introduced about cooperative evolution. Small variations in individual vigour or resources can lead to large variations in individual contributions to policing the group. Stronger individuals often invest all of their excess resources into policing, but weaker individuals do not contribute to group cohesion.