This article defends a version of the Westermarck Effect, integrating existing clinical, biological, and philosophical dimensions to incest avoidance. By focusing on care-based attachment in primates, my formulation of the effect suggests the power of a phylogenetic argument widely accepted by primatologists but not by cultural anthropologists. Identifying postadoption incest as a phenomenon with underexplored evidential value, the article sketches an explanatory strategy for reconciling the effect with the clinical reality of incest, concluding with an explicit argument against culture-first or (...) conventionalist accounts of incest avoidance prevalent in anthropology. (shrink)