Intrasexual selection for upper limb length in Homo sapiens

Sexual selection via contest competition has equipped countless organisms with weaponry in their appendages to overpower their opponents. Here, we tested the overarching hypothesis that the selection pressures created by intrasexual contest competition shaped humans’ upper limbs – specifically the length of their arms. We tested (1) whether greater arm length confers an advantage in contest competition among adult humans, (2) several possible means by which arm length might increase success in intrasexual contest competition, and (3) whether, consistent with male-male contest competition creating stronger selection pressures than female-female contest competition, male Homo sapiens have greater upper limb length. Our results provided unambiguous support for these hypotheses. First, we found that greater arm length is associated with increased success in intrasexual contest competition, an effect driven specifically by the capacity to knock opponents unconscious. Second, in diverse and demographically representative samples from four studies, we found unequivocal, cross-cultural evidence of previously undocumented sexual dimorphism in upper limb length: after controlling for allometry, across the globe, men exhibited longer upper limbs than women, from Croatian adolescents to older Singaporean adults to United States Army personnel (total N = 6,915) born across all major world regions (Africa, Europe, Asia, Oceania, and North, Central, and South America). The current studies introduce a previously undiscovered sexual dimorphism to the biological, anthropological, and psychological sciences: upper limb length.