Hadza Hunter-Gatherers and The Evolution of Human Cooperation: Evidence Against Partner Choice Models

Human cooperation is exceptional in the animal kingdom, and explaining its evolution is a puzzle. One hypothesis is that the ability to track others’ reputations and to choose our cooperative partners created biological markets, and competition within these markets selected for cooperators. Here, I test this hypothesis from the Hadza of Tanzania, one of the last remaining foraging populations. In Chapter 1, I use longitudinal data tracking cooperation in an economic game and residence patterns. In every year, contribution levels to the public good are similar within residence camps, fulfilling a necessary condition for the evolution of cooperation. However, cooperators in previous years were not more likely to live with cooperators in future years. Further, at the individual level, previous contributions did not predict future contributions. In Chapters 2 and 3, I use data from a ranking task in which Hadza ranked their campmates on character traits, hunting ability, and who they would like to live with in the future. In Chapter 2, I examine whether Hadza agree on perceptions of moral character. The Hadza disagree on which of their campmates exhibit moral character. The Hadza do agree though on what traits (e.g., generosity and hard work) contribute to overall moral character. These results indicate that the Hadza use similar criteria for evaluating moral character but do not agree on who exhibits these traits. The lack of agreement on perceptions of moral character may be due to the lack of stable moral dispositions among the Hadza. Finally, in Chapter 3, I examine which traits the Hadza prefer when choosing potential campmates. I find that the Hadza have only weak preferences to live with campmates that exhibit characters traits, and instead have stronger preferences to live with men who are better hunters. Further, there is no evidence that being a preferred campmate results in any benefits to one’s reproductive success, further undermining partner choice theories. Together, these results indicate that partner choice and other reputation-based strategies do not maintain cooperation among the Hadza, and more broadly, suggests that such mechanisms were not responsible for the evolution of human cooperation