We argue that grooming is a commodity that female primates can trade, either for itself or in exchange for other services (sensu biological markets theory) and that the decision to do either will depend on the degree of competition within a social group. We test this using data from four chacma baboon troops, living in two populations that di¡er markedly in the degree of contest competition. As predicted by the predominance of grooming dyads in which females are closely ranked there was, in all four troops, a positive correlation between the time invested by one partner and that by the other. In addition, as predicted, the allocation of time was more closely matched in troops where grooming could not be exchanged for anything else. In troops where resource competition was high, we found in one of two troops a positive relationship between rank distance and the discrepancy in time allocation, with the lower ranking of the partners contributing more grooming.

Why do people resort to plastic surgery to look young? Why are stepchildren at greatest risk of fatal abuse? Why do we prefer gossip to algebra? Why must Dogon wives live alone in a dark hut for five days a month? Why are young children good at learning language but not sharing? Over the past decade, psychologists and behavioral ecologists have been finding answers to such seemingly unrelated questions by applying an evolutionary perspective to the study of human behavior and psychology. Human Evolutionary Psychology is a comprehensive, balanced, and readable introduction to this burgeoning field. It combines a sophisticated understanding of the basics of evolutionary theory with a solid grasp of empirical case studies. Covering not only such traditional subjects as kin selection and mate choice, this text also examines more complex understandings of marriage practices and inheritance rules and the way in which individual action influences the structure of societies and aspects of cultural evolution. It critically assesses the value of evolutionary explanations to humans in both modern Western society and traditional preindustrial societies. And it fairly presents debates within the field, identifying areas of compatibility among sometimes competing approaches. Combining a broad scope with the more in-depth knowledge and sophisticated understanding needed to approach the primary literature, this text is the ideal introduction to the exciting and rapidly expanding study of human evolutionary psychology.

Human height is a highly variable trait, both within and between populations, has a high heritability, and influences the manner in which people behave and are treated in society. Although we know much about human height, this information has rarely been brought together in a comprehensive, systematic fashion. Here, we present a synthetic review of the literature on human height from an explicit evolutionary perspective, addressing its phylogenetic history, development, and environmental and genetic influences on growth and stature. In addition to presenting evidence to suggest the past action of natural selection on human height, we also assess the evidence that natural and sexual selection continues to act on height in contemporary populations. Although there is clear evidence to suggest that selection acts on height, mainly through life-history processes but perhaps also directly, it is also apparent that methodological factors reduce the confidence with which such inferences can be drawn, and there remain surprising gaps in our knowledge. The inability to draw firm conclusions about the adaptiveness of such a highly visible and easily measured trait suggests we should show an appropriate degree of caution when dealing with other human traits in evolutionary perspective.