This article explores the biocultural bases of spirit possession mediumship in the Afro-Brazilian religion, Candomblé. After a brief review of the literature, the article moves beyond the biomedical and social-structural explanations that have dominated the theoretical landscape, by attempting to construct an etiology of mediumship that is traced through the interface of individual characteristics with the cultural belief system that forms their context. Data were collected from a total of 71 individuals over the course of a year-long field study in Salvador, Brazil. Analyses of social ethnography, life history and semistructured interviews along with results from psychological inventories, suggest that altered states of consciousness should not be considered the central and defining element of mediumship. An alternative model is proposed, in which the combination of social conditions and somatic susceptibilities causes certain individuals to identify with the mediumship role, and predisposes them to dissociate. However in the context of Candomblé, dissociation is not a pathological experience, but rather a therapeutic mechanism, learned through religious participation, that benefits individuals with a strong tendency to somatize.