Third-party attitudes toward sibling incest: Evidence for Westermarck's hypotheses

Abstract

While substantial evidence supports the existence of Westermarck's [Westermarck, E. (1891). The history of human marriage. London: Macmillan & Co.] hypothesized inbreeding avoidance mechanism, questions remain. We examined the Westermarck hypothesis using third-party reactions to a fictional case of sibling incest, a method paralleling that of Lieberman et al. [Proc. R. Soc. Lond., B Biol. Sci. 273 (2003) 819]. Controlling for cultural attitudes, a history of cosocialization with an opposite-sex individual was associated with increased disgust at, and decreased tolerance of, others' incestuous behavior. Consistent with parental investment theory, this effect was stronger in females than in males. In males, each additional cosocialized sibling increased the strength of the response; in females, there was a nonsignificant trend in the same direction. Indirect measures failed to reveal a time-limited sensitive period during which cosocialization has maximal effect. These results both bolster the evidence in favor of Westermarck's inbreeding avoidance hypothesis and support his neglected explanation of the origin of incest taboos.