This study examines heartrate sharing in the context of a trust-building game. Through quantitative and qualitative analyses, we find that "elevated" (versus "normal") heartrate of an exchange partner is associated with negative mood attributions and reduced cooperation in a social dilemma game. To investigate how specific our findings are to heartrate (as opposed to some other "elevated" signal collected from the body), we replicate our initial experiment with an unfamiliar biosignal, "skin reflectivity". We find that both heartrate and the unfamiliar biosignal are associated with negative mood attributions, but we observe a decrease in cooperative behavior only with elevated heartrate. Qualitative results indicate that individuals may learn an association between our unfamiliar biosignal and the cooperative, trusting behavior of their partner. Our findings highlight the role prior beliefs can play in shaping interpretations of a biosignal, while suggesting that designers can, perhaps inadvertently, train users to associate signals with social meanings. We discuss implications for how wearable sensors can mediate social interactions.