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Abstract

Animals—human and nonhuman alike—rarely prefer close genetic relatives as sexual partners. Moreover, in the lab and in real life, humans typically react to thoughts of sex with their family members with extreme aversion. Scientists generally agree about why such aversions exist: The sexual avoidance of close genetic relatives evolved because the offspring of close relatives suffer increased risks of disease and death. But how do these sexual aversions develop? Recent research has addressed this question by focusing on people’s aversions to sex with their siblings. Researchers have discovered a set of cues people use during development to estimate whether someone in their social world is likely to be a sibling. When present, these cues typically lead to the development of intense sexual aversions. When the cues are absent, as they are when siblings are reared apart, kin-based sexual aversions tend not to develop. This article discusses the kinship cues regulating the development of sibling sexual aversions and recent work suggesting that the intensity of people’s sibling-directed sexual aversions colors their moral attitudes regarding the sexual behavior of others. Taken together, research on human inbreeding avoidance addresses fundamental questions pertaining to development and mate choice, as well as morality and social policy.

Keywords

inbreeding avoidance kin detection Westermarck effect

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It’s All Relative

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