Reciprocity stabilizes cooperation from the level of microbes all the way up to humans interacting in small groups, but does reciprocity also underlie stable cooperation between larger human agglomerations, such as nation states? Famously, evolutionary models show that reciprocity could emerge as a widespread strategy for achieving international cooperation. However, existing studies have only detected reciprocity-driven cooperation in a small number of country pairs. We apply a new method for detecting mutual influence in dynamical systems to a new large-scale data set that records state interactions with high temporal resolution. Doing so, we detect reciprocity between many country pairs in the international system and find that these reciprocating country pairs exhibit qualitatively different cooperative dynamics when compared to nonreciprocating pairs. Consistent with evolutionary theories of cooperation, reciprocating country pairs exhibit higher levels of stable cooperation and are more likely to punish instances of noncooperation. However, countries in reciprocity-based relationships are also quicker to forgive single acts of noncooperation by eventually returning to previous levels of mutual cooperation. By contrast, nonreciprocating pairs are more likely to exploit each other's cooperation via higher rates of defection. Together, these findings provide the strongest evidence to date that reciprocity is a widespread mechanism for achieving international cooperation.

中文翻译：

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在全球范围内检测互惠。

互惠会稳定合作，从微生物水平一直到人类以小组形式进行互动，但是互惠是否也构成了较大的人类群体（例如民族国家）之间稳定合作的基础？著名的进化模型表明，互惠可能会成为实现国际合作的广泛战略。但是，现有研究仅在少数几个国家/地区中发现了互惠驱动的合作。我们将一种用于检测动力学系统中相互影响的新方法应用于一个新的大规模数据集，该数据集可以以高时间分辨率记录状态相互作用。这样做，我们在国际体系中检测了许多国家/地区对之间的互惠性，发现与非双向国家/地区相比，这些双向国家/地区对国家的合作动力学表现出质的不同。与合作的进化理论相一致，往复的国家/地区对显示出更高水平的稳定合作，并且更有可能惩罚不合作的情况。但是，建立在互惠关系中的国家也可以更快地通过最终回到先前的相互合作水平来原谅单一的不合作行为。相比之下，非往复式对更可能通过更高的叛逃率来利用彼此的合作。这些发现共同提供了迄今为止最强有力的证据，即互惠是实现国际合作的广泛机制。