AbstractCooperation is pivotal for society to flourish and prosper. To ease cooperation, humans express and read emotions and intentions via explicit signals or subtle reflections of arousal visible in the face. Evidence is accumulating that humans synchronize these nonverbal expressions, as well as the physiological mechanisms underlying them, potentially promoting cooperative behavior. The current study is designed to verify the existence of this putative linkage between synchrony and cooperation. To that extent, 152 participants played multiple rounds of the Prisoner’s Dilemma game in a naturalistic dyadic interaction setting. During one round of games they could see each other, and during a second round they could not. The results showed that when people’s heart rate and skin conductance level aligned, they cooperated more successfully. Interestingly, for skin conductance level synchrony to boost cooperation, face to face contact was essential. The effect of heart rate synchrony on cooperation was context independent. Skin conductance level, but not heart rate, tended to closely correlate with changes in pupil size and we discuss how the pupil might provide a window to partners’ sympathetic arousal. These findings show how unconscious mechanisms guide social behavior and emphasize the importance of studying social processes between rather than within individuals in real-life interactions.Significance StatementHuman cooperation is remarkable in its scale, complexity, and efficiency. However, whereas people think of themselves as rational agents, they actually partly base their decision to cooperate with someone on gut feelings. These feelings are informed by non-verbal expressions that are picked up implicitly and that synchronize across interaction partners. For the first time, we show that the alignment of people’s arousal over multiple rounds of the Prisoners’ dilemma game predicts cooperative success. Through synchrony, partners converge emotionally, fostering understanding and bonding, which are key ingredients when it comes to successful cooperation. This suggests that successfully cooperating does not depend on individuals, but on the connection between individuals, emphasizing the importance of studying social decision-making processes in real-life settings.
