Beyond Westermarck: Can Shared Mothering or Maternal Phenotype Matching Account for Incest Avoidance?

Abstract

Westermarck's Hypothesis (WH) is widely accepted amongst evolutionary scientists as the best explanation of human incest avoidance. Although WH may account for incest avoidance between co-reared kin, it cannot explain other forms of incest avoidance, and therefore, cannot account for the differential incidence of sibling-sibling, mother-son, father-daughter and other forms of incest. WH also face problems adequately accounting for phenomena within its explanatory domain. Neither of the studies widely thought to corroborate WH (the Israeli kibbutz and Taiwanese simpua marriage studies) provides a genuine test of it, and the results of experimental thought to confirm WH are vitiated by methodological problems. The present paper considers two alternatives to WH: the shared mother hypothesis (SMH) and the maternal phenotype-matching hypothesis (MPMH). SMH states that human infants imprint on their mother, who then treat as kin those individuals towards whom their mother behaves in a kin-like or mate-like manner. MPMH states that humans unconsciously use the maternal phenotype as a visual template for estimating coefficients of relatedness, and that these estimate regulate altruistic and mating behavior. Both SMH and MPMH are able to account for the kibbutz and simpua marriage data, and entail additional epidemiological and experimental predictions. SMH and MPMH have greater explanatory power than WH, and MPMH has greater explanatory power than SMH.