Sharing resources between members of different tribes and collectives is common and well-documented. Surprisingly, little is known about factors that are conducive to building social relationships between groups. We design a common-pool resource experiment, where after harvesting, groups can send some of their harvest to augment the resource of the outgroup. We compare donations made by individuals collectively and independently of other group members, under the conditions of equal and unequal resources. We find that individuals acting as decision-makers, but not groups, donate harvests frequently even though it is payoff-reducing. We conduct an additional treatment, where each donation is matched (doubled) by an equivalent transfer of resources, making sharing between groups payoff-improving. Under matching donations, sharing between groups flourishes, but fails to prevent resource decline in most groups. Finally, our experiment reveals that members of low-endowment groups overharvest resources in expectation of donations from affluent groups, which leads to the tragedy of the commons.

不同部落和集体的成员之间共享资源是很常见的，而且有据可查。令人惊讶的是，人们对有利于在群体之间建立社会关系的因素知之甚少。我们设计了一个公共池资源实验，在收获后，群体可以发送一些收获来增加外群体的资源。我们在资源平等和不平等的条件下比较个人集体且独立于其他团体成员的捐赠。我们发现，作为决策者的个人（而不是团体）经常捐赠收成，尽管这会降低回报。我们进行了额外的处理，即每笔捐赠都会通过等量的资源转移来匹配（双倍），从而提高群体之间的共享回报。在匹配捐赠的情况下，群体之间的共享蓬勃发展，但未能阻止大多数群体的资源下降。最后，我们的实验表明，低捐赠群体的成员过度获取资源，期望富裕群体的捐赠，从而导致了公地悲剧。