Individual differences, the importance of which was identified by Darwin more than 150 years ago, are evident in multiple domains. This review discusses the role of temperament in health-related outcomes in rhesus monkeys. Temperament is proposed as affecting health outcomes via behavioral means, and also via physical means either through its direct association with variation in physiological systems (a “main effects” model), or through its impact on functioning when organisms are in stressful circumstances (an “interaction effects” model). The specific temperament factor described is Sociability, which reflects a tendency to affiliate, and which is associated with positive affect, and with differences in sensitivity of brain dopamine systems. Data are reviewed showing that individual differences in Sociability in rhesus monkeys are related to variation in sympathetic innervation of lymphoid tissue (a main effect), as well as to patterns of coping in socially stressful circumstances (an interaction effect). Results such as these have implications for studies in behavioral ecology, medicine, and even for management practices in captive colonies of nonhuman primates. Am. J. Primatol. 73:507–515, 2011. © 2010 Wiley-Liss, Inc.