Altruism among humans is common. It is especially prominent among in-group members. However, we are struck by results from laboratory-in-the-field experiments with out-group members of different ethnic or religious backgrounds. In all instances, the groups were rivals in civil wars. While we find almost no altruistic behavior among groups that were engaged in fighting, out-group altruism emerges with the passage of time. To address the underlying mechanism that might explain this puzzling result, we use a laboratory experiment that sorts between a norms revision explanation and a contact hypothesis explanation. Our findings show that contact with out-group members, in a mutually beneficial task, results in a reversion to prior levels of out-group altruism following conflict. Our findings reinforce the idea that deeply held attitudes toward a stigmatized out-group change following productive interactions with those out-group members. It is clear that hostility between groups need not persist over time.

人类之间的利他主义很普遍。这在组内成员中尤为突出。然而，我们对来自不同种族或宗教背景的外群体成员的现场实验室实验结果感到震惊。在所有情况下，这些团体都是内战中的竞争对手。虽然我们发现参与战斗的群体之间几乎没有利他行为，但随着时间的推移，群体外的利他主义出现了。为了解决可能解释这个令人费解的结果的潜在机制，我们使用了一个实验室实验，在规范修订解释和接触假设解释之间进行排序。我们的研究结果表明，在互惠互利的任务中与组外成员的接触会导致冲突后恢复到先前的组外利他主义水平。我们的研究结果强化了这样一种观点，即在与这些外群体成员进行富有成效的互动后，对被污名化的外群体变化的态度根深蒂固。很明显，群体之间的敌意不需要随着时间的推移而持续存在。