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We test two hypotheses, based on sexual selection theory, about gender differences in costly social interactions. Differential selectivity states that women invest less than men in interactions with new individuals. Differential opportunism states that women’s investment in social interactions is less responsive to information about the interaction’s payoffs. The hypotheses imply that women’s social networks are more stable and path dependent and composed of a greater proportion of strong relative to weak links. During their introductory week, we let new university students play an experimental trust game, first with one anonymous partner, then with the same and a new partner. Consistent with our hypotheses, we find that women invest less than men in new partners and that their investments are only half as responsive to information about the likely returns to the investment. Moreover, subsequent formation of students’ real social networks is consistent with the experimental results: being randomly assigned to the same introductory group has a much larger positive effect on women’s likelihood of reporting a subsequent friendship.
Reciprocal exchanges can be understood as the updating of an initial belief about a partner. This initial level of trust is essential when it comes to establishing cooperation with an unknown partner, as cooperation cannot arise without a minimum of trust not justified by previous successful exchanges with this partner. Here we demonstrate the existence of a representation of the initial trust level before an exchange with a partner has occurred. Specifically, we can predict the Investor’s initial investment—i.e. his initial level of trust toward the unknown trustee in Round 1 of a standard 10-round Trust Game—from resting-state functional connectivity data acquired several minutes before the start of the Trust Game. Resting-state functional connectivity is, however, not significantly associated with the level of trust in later rounds, potentially mirroring the updating of the initial belief about the partner. Our results shed light on how the initial level of trust is represented. In particular, we show that a person’s initial level of trust is, at least in part, determined by brain electrical activity acquired well before the beginning of an exchange.