Policing effectiveness depends on relatedness and group size

Cohesion of social groups requires the suppression of individual selfishness. Indeed, worker egg laying in insect societies is usually suppressed or punished
through aggression and egg removal. The effectiveness of such \"policing\" is expected to increase with decreasing relatedness, as inclusive fitness of group members is
more strongly affected by selfish worker reproduction when group members are less closely related to each other. As inclusive fitness is also influenced by the costs and
benefits of helping, the effectiveness of policing should decrease with increasing colony size, because the costs for the whole colony from selfish worker reproduction
are proportionally reduced in large groups. Here, we show that policing effectiveness in colonies of the ant Temnothorax unifasciatus is low in large groups and high in
small groups when relatedness is high. When we experimentally decreased the relatedness in groups, the policing effectiveness reached the same high level as i...