Altruism between close relatives can be easily explained. However, paradoxes arise when organisms divert altruism towards more distantly related recipients. In some social insects, workers drift extensively between colonies and help raise less related foreign brood, seemingly reducing inclusive fitness. Since being highlighted by W. D. Hamilton, three hypotheses (bet hedging, indirect reciprocity and diminishing returns to cooperation) have been proposed for this surprising behaviour. Here, using inclusive fitness theory, we show that bet hedging and indirect reciprocity could only drive cooperative drifting under improbable conditions. However, diminishing returns to cooperation create a simple context in which sharing workers is adaptive. Using a longitudinal dataset comprising over a quarter of a million nest cell observations, we quantify cooperative payoffs in the Neotropical wasp Polistes canadensis, for which drifting occurs at high levels. As the worker-to-brood ratio rises in a worker’s home colony, the predicted marginal benefit of a worker for expected colony productivity diminishes. Helping related colonies can allow effort to be focused on related brood that are more in need of care. Finally, we use simulations to show that cooperative drifting evolves under diminishing returns when dispersal is local, allowing altruists to focus their efforts on related recipients. Our results indicate the power of nonlinear fitness effects to shape social organization, and suggest that models of eusocial evolution should be extended to include neglected social interactions within colony networks.

近亲之间的利他主义很容易解释。然而，当生物体将利他主义转向更远亲的接受者时，就会出现悖论。在一些群居昆虫中，工蚁在群体之间广泛漂移，帮助饲养亲缘关系较弱的外来后代，这似乎降低了包容性。自从被 WD Hamilton 强调以来，针对这种令人惊讶的行为提出了三个假设（赌注对冲、间接互惠和合作收益递减）。在这里，我们使用包容性适应度理论表明，赌注对冲和间接互惠只能在不太可能的条件下驱动合作漂移。然而，合作收益递减创造了一个简单的环境，在这个环境中，共享员工具有适应性。使用包含超过四分之一百万巢细胞观察结果的纵向数据集，Polistes canadensis，其漂移发生在高水平。随着工蚁所在群体中工蚁与育雏比率的上升，工蚁对预期群体生产力的预测边际收益会减少。帮助相关群体可以将精力集中在更需要照顾的相关群体上。最后，我们使用模拟表明，当分散在本地时，合作漂移会在收益递减的情况下演变，从而使利他主义者能够将他们的努力集中在相关的接受者身上。我们的结果表明非线性适应性效应对塑造社会组织的力量，并建议应该扩展真社会进化模型以包括群体网络中被忽视的社会互动。