Oxytocin-mediated empathy constrains “free-riding” and stabilizes reciprocal cooperation in rats

Cooperation in nature can yield immediate mutual benefits or depend on delayed reciprocity. The latter is vulnerable to “free-riding,” where a beneficiary fails to return help, and classical theories emphasize external enforcement (e.g., second- and third-party punishment). Yet such mechanisms are limited in real-world animal settings.

In Science Bulletin, researchers led by Zuoren Wang (CEBSIT, Chinese Academy of Sciences) and Yulong Li (Peking University) present a fully automated reciprocal cooperation task in rats. Two rats, placed in adjacent chambers, must poke their respective ports within a 1-second window to complete a cooperative trial; only one receives water, and the beneficiary alternates randomly across trials—creating a “pay-first, reward-later” structure. With training, rats showed robust direct reciprocity rather than mere mutualism, and richer social interaction predicted faster, more successful cooperation; adding a transparent barrier that dampened interaction reduced reciprocal performance.

The team then asked whether reciprocity experience shapes empathy, assessed via an observational fear paradigm. Compared with individually trained controls, reciprocity-trained rats displayed higher emotional contagion toward their partners whether co-housed or separately housed—while demonstrators’ shock responses, observers’ social attention, and 22-kHz calls did not differ between groups, arguing for a specific effect of reciprocity experience.

To probe mechanism, the authors combined GRAB-OXT1.0 with fiber photometry recording and found markedly higher oxytocin release in the orbitofrontal cortex (OFC) during reciprocity than during mutualism or individual tasks. More importantly, oxytocin-deficient (OXT-KO) rats free-ride more often, were less likely to reciprocate after betrayal, and failed to show the empathy increase seen in wild-type animals—implicating oxytocin-mediated empathy as a key internal brake on free-riding that stabilizes cooperation. Together, the findings position OFC oxytocin signaling and empathy as an intrinsic motivational pathway complementing external enforcement models of cooperation.

Corresponding Authors: Yulong Li; Zuoren Wang
First Author: Miaoyaoxin Wang
Funding: Science and Technology Innovation 2030 Major Project (2022ZD0205100); Strategic Priority Research Program of the Chinese Academy of Sciences (XDB32010300); Shanghai Municipal Science and Technology Major Project (2018SHZDZX05).