Having other-sex siblings predicts moral attitudes to sibling incest, but not parent-child incest

Moral opposition to incest is thought to play an important role in preventing inbreeding. Some researchers have proposed that moral opposition to sibling incest is greater for individuals who have other-sex siblings. Empirical evidence for this claim is mixed, however. Consequently, we compared moral opposition to both third-party sibling and third-party parent-child incest in participants with other-sex siblings only (N=313) and participants with own-sex siblings only (N=269). Having other-sex siblings predicted moral attitudes to sibling, but not parent-child, incest. These results support the proposal that moral opposition to sibling incest is sensitive to aspects of family structure, specifically the sex of siblings.