Among species expressing bi-parental care, males' testosterone is often low when they cooperate with females to raise offspring. In humans, low testosterone men might have an advantage as nurturant partners and parents because they are less prone to anger and reactive aggression and are more empathetic. However, humans engage in cooperative, supportive relationships beyond the nuclear family, and these prosocial capacities were likely critical to our evolutionary success. Despite the diversity of human prosociality, no prior study has tested whether men's testosterone is also reduced when they participate in emotionally supportive relationships, beyond partnering and parenting. Here, we draw on testosterone and emotional social support data that were collected from older men (n = 371; mean: 61.2 years of age) enrolled in the National Health and Nutrition Examination Survey, a US nationally-representative study. Men who reported receiving emotional support from two or more sources had lower testosterone than men reporting zero support (all p < 0.01). Males with the most support (4+ sources) also had lower testosterone than those with one source of support (p < 0.01). Men who reported emotional support from diverse (kin + non-kin or multiple kin) sources had lower testosterone than those with no support (p < 0.05). Expanding on research on partnering and parenting, our findings are consistent with the notion that low testosterone is downstream of and/or facilitates an array of supportive social relationships. Our results contribute novel insights on the intersections between health, social support, and physiology. (C) 2015 Elsevier Inc. All rights reserved.