For some time Webster's Duchess of Malfi has been interpreted by reference to brother-sister incest, but that explanation has not been well integrated with other concerns in the play, nor has its sheer presence been questioned. Anthropological kinship theory, however, which conceives incest as a social act, reveals relations among the brother-sister plot, the play's major thematic element of social mobility, and the Jacobean setting from which the theme arose. Seen in this anthropological light, social-structural relations come into view among Ferdinand's incestuous inclination, his sister's cross-class marriage, and Antonio's and Bosola's upward social mobility. These relations in turn show how the play is grounded in its particular historical setting, at a time of substantial changes in notions of social role, changes that helped make visible the social determination of personal identity.