This thesis consists of three papers that explore early human organization. In the first paper I argue that the economic and social structure of early humans would have resulted in an especially difficult consanguinity problem. In particular, adverse selection in the exogamous marriage market would have resulted in high levels of consanguinity and resulting fitness depression. A partial solution to this problem was the evolution of aversion to endogamy, known as the Westermarck effect, and was essential for the survival of our species. The second paper (joint with Haiyun Chen) develops a model that explains linguistic diversity as the cumulative result of strategic incentives faced by linguistic groups. In this model, autonomous groups interact periodically in games that represent either cooperation, competition, or a lack of interaction. Common language facilitates cooperation such as trade, whereas language unique to one group affords that group an advantage in competitive interactions. The relative frequency of cooperation and conflict in a region provides incentives for each group to modify their own language, and therefore leads to changes in linguistic diversity over time. Our model predicts that higher frequency of conflict relative to cooperation will increase a region's linguistic diversity. The third paper (joint with Gregory K. Dow and Clyde G. Reed) investigates the incidence of early warfare among foragers and farmers in prehistory. Our focus is specifically on conflict over land. Food is produced using inputs of labor and land, and the probability of victory in a conflict depends on relative group sizes. The group sizes are determined by individual migration and Malthusian population dynamics. Both factors result in larger populations at better sites, which deters attack. There are two necessary conditions for warfare: high enough individual mobility costs and large enough shocks to the relative productivities of the sites. Together, these conditions are sufficient. In particular, technological or environmental shocks that alter the productivities of sites can trigger warfare, but only if individual agents do not change sites in response. These results are consistent with evidence from archaeology and anthropology.
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Thesis advisor: Robson, Arthur
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