Physiological synchrony predicts reduced reciprocity only in face-to-face interactions

Across species, cooperative decisions are often asymmetric: one individual risks their own resources before knowing whether others will reciprocate. Such uncertainty is central to the evolution of trust and reciprocal exchanges. Physiological synchrony is frequently assumed to support cooperation, yet it remains unclear both how synchrony arises and how it functions in these asymmetric social interactions. We tested whether mutual visibility shapes physiological synchrony, and whether synchrony in turn predicts cooperative behaviour in repeated trust-reciprocity exchanges. Skin conductance synchrony was stronger and more temporally aligned when individuals interacted face-to-face, yet facial expressivity did not account for this effect. Contrary to the dominant perspective, higher synchrony predicted reduced reciprocity, and only when partners could see each other. Exploratory analyses suggested that this negative link depended on arousal, indicating that the social function of synchrony is shaped by both mutual visibility and the affective state of the individual involved. These findings show that physiological synchrony is not a uniformly prosocial mechanism but a context-dependent phenomenon that can undermine cooperation under certain conditions, offering insight into the mechanisms and the function of synchrony in shaping cooperative exchanges.