Experiencing an abundance of positive and negative emotions (emodiversity) can predict better outcomes than positive or negative emotions in separation. We tested whether state and trait emodiversity influence behavior in challenging social situations and whether this link is mediated by parasympathetic activation. Two hundred ten undergraduate students reported their emotions from the preceding month and were then randomly assigned to one of four movie-clip conditions: emodiverse (varied positive and negative emotions), positive emotions, negative emotions, or neutral. Afterward, participants engaged in an ultimatum game involving an uneven monetary split. They had to decide either to accept the offer, which would be a prosocial act financially benefit the proposer, or reject it, which would financially harm the proposer. Electrocardiogram, hemodynamic, and respiratory response were recorded continuously. We found that emodiversity, whether temporary or long-term, did not affect participants’ social decisions or physiological responses. Contrary to the emodiversity hypothesis, we found more straightforward effects. First, inducing negative emotions adversely affected participants’ affect and increased their likelihood of rejecting the offer. Second, participants with higher positive affect, greater parasympathetic activation, and faster breathing rates were more likely to accept the offer. In summary, we found little support for the proposed benefits of emodiversity. Instead, our findings highlighted well-established affective and physiological effects (i.e., parasympathetic activation), demonstrating that positive emotions were associated with increased prosocial behavior, whereas negative emotions reduced it. (PsycInfo Database Record (c) 2026 APA, all rights reserved)