Organisms often respond in ways that appear to benefit others rather than themselves. This phenomenon is consistent with the views of Darwin (1859) and Dawkins (1999) that individuals may exploit the responses of others. This phenomenon, “social parasitism”, has been extensively investigated in social insects, particularly, ants. Other empirical studies have demonstrated social parasitism in fish, birds, and mammals. This paper reviews several possible examples of mammalian social parasitism, with an emphasis upon intraspecific social parasitism (ISP) in Neotropical primates. Social parasitism is discussed as a life history feature of long-lived, social organisms such as many primates, including humans. A simple mathematical model, applied to social parasitism, is presented linking parasite transmission to a parasite’s influence on its host. Phenotypic manipulation is assessed as a mechanism of social parasitism, and possible examples from the literature on Neotropical primates are provided. Social parasitism is discussed in relation to the evolution of higher grades of sociality (eusociality, cooperative breeding), manipulation success (infectivity), and the evolution of virulence (e.g., aggression, punishment). It is proposed that an understanding of variations in virulence and infectivity by social parasites is likely to reveal important evolutionary dynamics for an integrated view of social evolution.