Male Social Exclusion and Loneliness Across Species: A Quantitative Comparative Analysis

Male social exclusion is pervasive across mammalian species. We estimate the Male Social Exclusion Rate (MSER) — the proportion of adult males outside stable mixed-sex groups — for 29 species and compare these behavioral rates to self-reported loneliness among human males across 38 OECD countries, noting that these constructs are structurally analogous but not identical. Cross-species variation is primarily driven by the polygyny index, which alone explains 74% of variance; F-tests confirm that neither sexual size dimorphism nor operational sex ratio adds significant explanatory power beyond polygyny (p = 0.20, p = 0.42). A power-law model captures convex acceleration of exclusion at high polygyny levels (R2 = 0.84). Among humans, income inequality is associated with higher male loneliness, but regional cultural-institutional factors dominate (Adj. R2 rises from 0.22 to 0.66 with region fixed effects; LOO-CV R2 = 0.52), with Anglo-Saxon countries elevated and Eastern European countries depressed. Time series analysis (2006-2024) reveals young male loneliness increasing at approximately 0.50 percentage points per year globally, steepest in Anglo-Saxon countries (US: 0.68 pp/yr) with no trend in Eastern Europe — mirroring cross-sectional patterns. Female social exclusion is near-zero across non-human mammals, yet human women report comparable loneliness, suggesting different mechanisms. Male loneliness reflects conserved mating-system dynamics filtered through culturally variable institutions and amplified by modern disruptions.