Human society operates on large-scale cooperation. However, individual differences in cooperativeness and incentives to free ride on others' cooperation make large-scale cooperation fragile and can lead to reduced social welfare. Thus, how individual cooperation spreads through human social networks remains puzzling from ecological, evolutionary, and societal perspectives. Here, we identify oxytocin and costly punishment as biobehavioral mechanisms that facilitate the propagation of cooperation in social networks. In three laboratory experiments (n = 870 human participants: 373 males, 497 females), individuals were embedded in heterogeneous networks and made repeated decisions with feedback in games of trust (n = 342), ultimatum bargaining (n = 324), and prisoner's dilemma with punishment (n = 204). In each heterogeneous network, individuals at central positions (hub nodes) were given intranasal oxytocin (or placebo). Giving oxytocin (vs matching placebo) to central individuals increased their trust and enforcement of cooperation norms. Oxytocin-enhanced norm enforcement, but not elevated trust, explained the spreading of cooperation throughout the social network. Moreover, grounded in evolutionary game theory, we simulated computer agents that interacted in heterogeneous networks with central nodes varying in terms of cooperation and punishment levels. Simulation results confirmed that central cooperators' willingness to punish noncooperation allowed the permeation of the network and enabled the evolution of network cooperation. These results identify an oxytocin-initiated proximate mechanism explaining how individual cooperation facilitates network-wide cooperation in human society and shed light on the widespread phenomenon of heterogeneous composition and enforcement systems at all levels of life.

SIGNIFICANCE STATEMENT Human society operates on large-scale cooperation. Yet because cooperation is exploitable by free riding, how cooperation in social networks emerges remains puzzling from evolutionary and societal perspectives. Here we identify oxytocin and altruistic punishment as key factors facilitating the propagation of cooperation in human social networks. Individuals played repeated economic games in heterogeneous networks where individuals at central positions were given oxytocin or placebo. Oxytocin-enhanced cooperative norm enforcement, but not elevated trust, explained cooperation spreading throughout the social network. Evolutionary simulations confirmed that central cooperators' willingness to punish noncooperation allowed the permeation of the network and enabled the evolution of cooperation. These results identify an oxytocin-initiated proximate mechanism explaining how individual cooperation facilitates network-wide cooperation in human social networks.

人类社会在大规模合作中运作。然而，合作的个体差异和搭便车的动机使大规模合作变得脆弱，并可能导致社会福利下降。因此，从生态、进化和社会的角度来看，个体合作如何通过人类社会网络传播仍然令人费解。在这里，我们将催产素和代价高昂的惩罚确定为促进社交网络合作传播的生物行为机制。在三个实验室实验中（n = 870 名人类参与者：373 名男性，497 名女性），个体被嵌入异构网络中，并在信任游戏（n = 342）、最后通牒谈判（n= 324), 和带有惩罚的囚徒困境 ( n= 204）。在每个异构网络中，位于中心位置（中心节点）的个体被给予鼻内催产素（或安慰剂）。给予中心个体催产素（对比安慰剂）增加了他们对合作规范的信任和执行。催产素增强了规范执行，但没有提高信任，这解释了合作在整个社交网络中的传播。此外，基于进化博弈论，我们模拟了在异构网络中交互的计算机代理，其中心节点在合作和惩罚级别方面各不相同。仿真结果证实，中央合作者惩罚不合作的意愿允许网络渗透并促成网络合作的演化。

重要性声明人类社会在大规模合作中运作。然而，由于搭便车可以利用合作，因此从进化和社会的角度来看，社交网络中的合作是如何出现的仍然令人费解。在这里，我们将催产素和利他惩罚确定为促进人类社交网络合作传播的关键因素。个体在异质网络中反复玩经济游戏，处于中心位置的个体被给予催产素或安慰剂。催产素增强了合作规范的执行，但没有提高信任，这解释了合作在整个社交网络中的传播。演化模拟证实，中央合作者惩罚不合作的意愿允许网络渗透并促成合作的演化。