Hominin Population Structure, Mating Systems, and Intrasexual Competition : Implications for Craniofacial Robusticity and the Self-Domestication Hypothesis.

The self-domestication hypothesis has made significant contributions to our thinking about hominin evolution and ecology. It has struggled, however, to find compelling and testable causes of reductions in hominin aggression and violent behavior. This paper examines variability among hominin mating systems and imbalances in operation sex ratios (OSRs) as a potential factor influencing levels of aggression resulting from male intrasexual competition, i.e., male-male competitive aggression. This paper uses multivariate generalized linear modeling (GLM) to examine data from modern hunter-gatherer societies having to do with the causes and consequences of OSR imbalances (as reflected by levels of polygamy) and male intrasexual competition. This paper focuses especially on the role of population density as a potential source of OSR variability. This paper shows that population density correlates strongly with hunter-gatherer OSRs and that this relationship remains strong when controlling for the effects of other potentially intercorrelated variables. This paper shows that, among modern hunter-gatherers, lower population densities lead to greater imbalances in terms of OSRs and therefore higher levels of male-male aggression. This implies high levels of male intrasexual competition among early hominins given likely conditions of very low population densities. The paper closes by proposing some strategies for investigating hominin demographic patterning prehistorically and predicting OSR imbalances, as well as intrasexual competition, based on that information.