Intrasexual and intersexual selection for neck musculature in men

Over 150 years ago, Darwin (1871) argued that humans evolved to inflict and resist damage in violent combat. Until now, however, no research has examined the claim that humans have evolved anatomical structures to both better inflict and resist damage in real-world agonistic exchanges. Human neck musculature has long been theoretically implicated in force output and concussion resistance capacities, and we proposed that human neck musculature evolved to increase damage resistance (knockout resistance) and damage infliction (knockout power) in real-world violent combat. Larger neck musculature was indeed associated with greater fighting success, knockout resistance, and knockout power capabilities across 715 professional combatants, after comprehensively controlling for weight, height, age, fighter's debut date, sex, reach, leg reach, and facial structure (Study 1). We then discovered that human neck musculature is one of the most sexually dimorphic human anatomical features, when compared against over 70 other human anatomical features (Study 2; N = 6000). We then found that human psychological systems have evolved to generate social perceptions in response to neck musculature. Men with greater physiological neck strength are perceived as stronger fighters (Study 3, after controlling for weight, height, age, beardedness, and torso, arm, bicep, and lower body strength). Study 4 specifically showed that men with larger sternocleidomastoid and upper trapezius muscles -- muscles implicated in damage resistance and infliction, respectively -- are perceived as more dominant (i.e., strong, masculine, anger-prone, and aggressive) and attractive (i.e., short and long-term attractiveness). Our results comprehensively demonstrate that human neck musculature evolved to increase resource-holding potential in violent combat, for which human psychological systems have consequently evolved an attraction. Given that human neck musculature is novel to the psychological and biological sciences, implications are discussed for the research areas of formidability assessment, contest competition, violence, face perception, emotion recognition, attraction and relationship research, political and organisational psychology, sports performance, and biomedical research.