Parochial altruism and the absence of the group size paradox in inter-group conflicts

Experiments on contests between groups typically find that large groups are more likely to win than small groups even if theory predicts otherwise. One explanation in the literature is parochial altruism: altruism towards members of the in-group and hostility towards members of the out-group. We model a mixed contest - a combination of a group contest and an individual contest - in which parochial altruism is predicted to have no impact on the groups` odds. Preliminary experimental evidence suggests that - contrary to theory - the large group is substantially more likely to win the group contest than the small group. Hence, forces beyond parochial altruism counteract the group size paradox. As deviations from theory diminish over time those other forces seem to be transient.