Prosocial preferences can escalate intergroup conflicts by countering selfish motivations to leave.

When defending against hostile enemies, individual group members can benefit from others staying in the group and fighting. However, individuals themselves may be better off by leaving the group and avoiding the personal risks associated with fighting. While fleeing is indeed commonly observed, when and why defenders fight or flee remains poorly understood and is addressed here with three incentivized and preregistered experiments (total n = 602). In stylized attacker-defender contest games in which defenders could stay and fight or leave, we show that the less costly leaving is, the more likely individuals are to abandon their group. In addition, more risk-averse individuals are more likely to leave. Conversely, individuals more likely stay and fight when they have pro-social preferences and when fellow group members cannot leave. However, those who stay not always contribute fully to group defense, to some degree free-riding on the efforts of other group members. Nonetheless, staying increased intergroup conflict and its associated costs.