Highlights d Assortment on cooperation is a characteristic feature of hunter-gatherer life d Assortment persists despite substantial migration and residential mixing d No evidence for stable social types or a preference to live with cooperators d Individuals respond in kind to the cooperative behavior of their group members

SUMMARYWidespread cooperation is a defining feature of human societies from hunter-gatherer bands to nation states. But explaining its evolution remains a challenge. While positive assortment – of cooperators with cooperators – is recognized as a basic requirement for the evolution of cooperation, the mechanisms governing assortment are debated. Moreover, the social structure of modern hunter-gatherers, characterized by high mobility, residential mixing and low genetic relatedness, undermine assortment and add to the puzzle of how cooperation evolved. Here, we analyze four years of data (2010, 2013, 2014, 2016) tracking residence and levels of cooperation elicited from a public goods game (PG), in Hadza hunter-gatherers of Tanzania. Data were collected from 56 camps, comprising 383 unique individuals, 137 of whom we have data for two or more years. Despite significant residential mixing, we observe a robust pattern of assortment necessary for cooperation to evolve: In every year, Hadza camps exhibit high between-camp and low within-camp variation in cooperation. We further consider the role of homophily in generating this assortment. We find little evidence that cooperative behavior within individuals is stable over time or that similarity in cooperation between dyads predicts their future cohabitation. Both sets of findings are inconsistent with homophilic models that assume stable cooperative and selfish types. Consistent with social norms, culture and reciprocity theories, the data suggest that the strongest predictor of an individual’s level of cooperation in any given year is the mean cooperation of their campmates in that year. These findings underscore the adaptive nature of human cooperation – particularly its responsiveness to social contexts – as a feature important in generating the assortment necessary for cooperation to evolve.

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In “Improving the Chilean College Admissions System,” Rios, Larroucau, Parra, and Cominetti describe the design and implementation of a new system to solve the Chilean college admissions problem. The authors develop an algorithm that (i) obtains all applicant/program pairs that can be part of a stable allocation when preferences are not strict and when all students tied in the last seat of a program (if any) must be allocated and (ii) efficiently incorporates affirmative action, which is part of the system to correct the inefficiencies that arise from having double-assigned students. By unifying the regular admission with affirmative action, the solution proposed and implemented by the authors has improved the allocation of approximately 2.5% of students assigned every year since 2016, helping to improve the overall efficiency of the system.