The "Social Brain," Reciprocity, and Social Network Segregation along Ethnic Boundaries.

How does segregation along ethnic boundaries emerge in social networks? Human evolution resulted in highly social beings, capable of prosociality, mindreading, and self-control, which are important aspects of the "social brain." Our neurophysiologically "wired" social cognition implies different cognitive goal frames. In line with recent developments in behavioral theory, the present study defines network ties as episodes of social exchange. This dynamic definition can account for shifts in goal frames during an exchange episode: whereas deliberate choice and hedonic or gain goals drive the initiation of a tie, given the opportunity structure, the normative goal frame activates a strong dynamic effect of reciprocity, which limits actors' choice set and appears as "self-organization" at the network level. Longitudinal analyses of 18 birthday party networks comprising 501 students support the definition of network ties as exchange episodes, as well as the relevance of humans' inherent tendency to reciprocate. However, reciprocation is much stronger in dyads of the same ethnicity than in dyads of different ethnicities. Network segregation along ethnic boundaries results from deliberate decisions during the initiation of an episode, but also from different commitments to reciprocity during the ongoing exchange process, depending on intra or interethnic dyadic constellations.