Are affines trated as biological Kin? a test of Hughes's hypothesis

Affines (or "in-laws") have long been recognized within anthropology as a special kind of kin. Evolutionary biology, in contrast, has typically treated affines as though they were unrelated: only direct genetic kinship counts. However, Hughes (1988) argued that Hamilton's concept of inclusive fitness naturally includes affinal kin as kin because true kin and their affines share genetic interests in future generations. We test this proposal by asking whether affinal relatives are treated more like biological kin or unrelated friends in terms of perceived emotional closeness. We show for a sample of contemporary Belgians that affines are indeed treated more or less the same as biological kin of similar nominal relatedness and not at all like unrelated friends. These findings suggest that Hughes was right in his reinterpretation of Hamilton and that affinal kinship needs to be considered in biological studies of human kinship.