Combined, our results provide comprehensive support for the argument that intrasexual selection has uniquely shaped Homo sapiens upper limb length to enhance fighting performance in real-world combat

Caton, Neil R., and David M. G. Lewis. 2021. “Armed Forces: Intrasexual Selection for Upper Limb Length in Homo Sapiens.” PsyArXiv. August 31. doi:10.31234/osf.io/fw6s9

Abstract: Numerous taxa have evolved physiological appendage-based weaponry to increase damage output in violent fights, but no research has empirically shown that Homo sapiens upper appendages have uniquely evolved to increase resource-provisioning potential in real-world combat. In Study 1, we used actual fight outcome data (N = 715 fighters) to examine multiple competing hypotheses—the striker, defender, grappler, and knockout hypotheses—for the evolution of Homo sapiens upper limb length, controlling for approximately a dozen confounding variables (e.g., biacromial width, lower limb length, age, weight, height). There was exclusive support for the knockout hypothesis: upper limb length increases fighting success through knockout power. There was also evidence for a real-world association between biacromial width and knockout power. Because sexual dimorphism often emerges from selection on morphological structures that improve male’s fighting success, we consequently expected sexual dimorphism in upper limb length. Studies 2a-2d provided powerful evidence for this new universal sexual dimorphism in upper limb length. Even after controlling for weight, height, and lower limb length, males exhibited longer upper limbs than females across the globe: from mixed-martial-artists (Study 2a) and Croatian adolescents (but not pre-pubertal children; Study 2b) to older Singaporean adults (Study 2c) and over 6,000 United States Army personnel (Study 2d) born across seven major world regions (Africa, Europe, Asia, Oceania, and North, Central, and South America). Combined, our results provide comprehensive support for the argument that intrasexual selection has uniquely shaped Homo sapiens upper limb length to enhance fighting performance in real-world combat.