Westermarck’s Hypothesis is widely accepted by evolutionary scientists as the best explanation for human incest avoidance. However, its explanatory shortcomings have been largely ignored and it has never been pitted against alternative biological hypotheses. Although WH may account for incest avoidance between co-reared kin, it cannot explain other forms of incest avoidance, and cannot account for the differential incidence of sibling-sibling, mother-son, father-daughter and other forms of incest. WH also faces problems adequately addressing phenomena within its explanatory domain. Neither of (...) the studies generally considered to corroborate WH provides a genuine test of it, and the results of experiments thought to confirm WH are vitiated by methodological problems. The present article considers two alternatives to WH: the shared mother hypothesis and the maternal phenotype-matching hypothesis . SMH states that human infants imprint on their mother, and then treat as kin those individuals toward whom their mother behaves in akin-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 estimates 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. (shrink)