Shared memberships, social statuses, beliefs, and places can facilitate the formation of social ties. Two-mode projections provide a method for transforming two-mode data on individuals’ memberships in such groups into a one-mode network of their possible social ties. In this paper, I explore the opposite process: how social ties can facilitate the formation of groups, and how a two-mode network can be generated from a one-mode network. Drawing on theories of team formation, club joining, and organization recruitment, I propose three models that describe how such groups might emerge from the relationships in a social network. I show that these models can be used to generate two-mode networks that have characteristics commonly observed in empirical two-mode social networks and that they encode features of the one-mode networks from which they were generated. I conclude by discussing these models’ limitations and future directions for theory and methods concerning group formation.

The social behaviour, ranging, and stereotypic behaviours of four red ruffed lemurs (one female, three males) was observed during group formation and release into a 2.25ha natural habitat enclosure at the Duke University Primate Center (DUPC). The female was immediately dominant to all males and there was no female-male affiliation during the initial stages of group formation. The group became identifiable as a unit after release to free-ranging when affiliation and group vocalizations began. Affiliation and vocalizations continued during subsequent recagings. Male dominance rank reflected relative age, but was subject to reversals. The stresses involved in release and group formation, however, can temporarily produce new aberrant behaviours which are soon replaced by normal behaviours. Once released into the large enclosure, stereotypic behaviours became infrequent but did not disappear. Other novel behaviours such as catatonic huddle and all male huddles were observed during release. Natural habitat enclosures can be important tools in the psychological well-being of captive primates.

We develop a theory of state formation shedding light on the rise of the first stable state institutions in Bronze Age Mesopotamia. Our analysis suggests that the mix of adverse production conditions and unforeseen innovations pushed groups favored by old technologies to establish the state by granting political and property rights to powerless individuals endowed with new and complementary skills. Through these reforms, the elite convinced the nonelite that a sufficient part of the returns on joint investments would be shared via public spending and, thus, to cooperate and accumulate a culture of cooperation. Different from the main alternative theories, we stress that: (1) group formation is heavily shaped by unforeseen shocks to the returns on both risk-sharing and innovation; (2) complementarity in group-specific skills, and not violence, is key determinant of state formation; (3) military, merchant and, especially, religious ranks favored state formation and culture accumulation.