富裕層の相続をめぐる競争が、文化集団淘汰ではなく、一夫一婦制の進化を促進する

The decline of polygyny and the rise of monogamy among complex, stratified societies characterized by high wealth inequality present a long-standing puzzle in anthropology. Competing explanations suggest that monogamy either i) reduces reproductive inequality, fosters cooperation, and enhances success in intergroup competition, or ii) mitigates conflict over heritable wealth-especially land-under conditions of high social stratification and ecological constraints. Both frameworks are influenced by the history of Indo-European societies, where monogamy has long been normative and closely associated with land inheritance and state formation. However, normative monogamy is also found in many societies beyond this historical context. To evaluate these competing hypotheses, we formalized their causal structure and applied Bayesian phylogenetic multilevel models to a global sample of 186 societies. Our results show that monogamy is strongly associated with land privatization and, in some regions, with ecological or demographic proxies for land scarcity-supporting the view that competition over heritable wealth promotes monogamy globally. In contrast, support for the intergroup competition model is inconsistent: While it may explain monogamy in some language families, these dynamics do not extend to most societies in our sample. Our findings suggest that monogamy arose repeatedly under similar socioecological conditions and cannot be fully explained by theories based primarily on Indo-European history.