Recent changes in British law have enabled adults who were adopted when young to trace their biological relatives. The reported frequency of incestuous sentiments and relationships in these reunions, together with the individual experiences and the meanings which are attached to them, challenge fundamental assumptions of both Westermarck's and Freud's theories. Post-adoption incest is characterized experientially by a romantic search for attachment followed by a recognition of oneself in the other. The question of incest has been fundamental to biosocial theories of individual and cultural development. Some revisions are proposed, principally that adult sexual interests remain constrained both by early attachments and later phenotypic matching, reinforced by personal contingencies and cultural rules.