Adaptations for contest competition in human males

Abstract

There is mounting evidence that generations of violent contest competition have shaped the bodies and behaviours of human males. This thesis tests several intrasexual selection hypotheses from the evolutionary social science literature, proposes alternatives and formulates novel ones. First, I investigate the individual differences that underpin how men perceive dominance in other men. Across 3 studies, I find that young, formidable men show better performance on a test of dominance sensitivity using transformed faces of other men. Second, across 3 more studies I fail to find evidence that men with more aggressive-looking faces or higher facial width-to-height ratios (a proposed evolved signal of threat) are more successful fighters. I discuss foundational questions that future studies in this area might address. Third, I propose the hypothesis that men show disproportionately long arms compared to women because longer arms provide an advantage in fighting, and I review the literature and offer data to support this. Fourth, I provide support to the Fighting hypothesis of left-handedness, which argues that one reason non-right-handedness is sustained in human populations despite fitness costs is because it provides a negative frequency-dependent advantage in fighting. Finally, I conduct a study to rule out an alternative mechanism that explains left-handers' increased fighting success, by testing whether steroid hormone levels differ by handedness. I find that left, right and mixed-handers all show similar levels of testosterone and oestradiol. Notably, all of my studies are the largest to address their respective questions. Further, they employ modern, robust statistical methods such as mixed-effects models, Bayesian regressions and bootstrap tests. In my general discussion I discuss the meta themes of my research, including the importance of large samples and replication studies, the value of varied sources of data for the social sciences, and the perils of evolutionary social scientists over-inferring adaptations where they may not exist. I also discuss what I see to be fruitful future directions for the study of intrasexual competition in males, including a de-emphasis on studies of face shape and static social encounters, and a focus on coalitional violence and contest competitions over time. I believe my thesis illustrates the promise and challenges of an evolutionary approach to social science

Date of Award	31 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Chris Klingenberg (Supervisor), Robert Gilman (Supervisor) & Andrew Chamberlain (Supervisor)