Mapping the cognitive architecture of systems for kin detection and inbreeding avoidance: The Westermarck hypothesis and the development of sexual aversions between siblings

The avoidance of sexual relations with close genetic relatives was an important and recurrent selection pressure throughout our evolutionary history due to the increased probability of expressed deleterious recessives and the decreased resistance to pathogens. These enduring selection pressures would have selected for cognitive mechanisms designed to detect close kin, assess degrees of genetic relatedness, and use this information to inhibit sexual relations between them during adulthood. The detection of close kin relies on the presence of evolutionarily stable cues that distinguished kin from non-kin and distinguished among different classes of genetic relatives. As initially proposed by Edward Westermarck, childhood co-residence would have cued patterns of relatedness for a specific class of close kin, siblings. Whereas natural populations with exceptional rearing practices have provided support for the Westermarck Hypothesis, very little is known about the psychological architecture of human kin detection and inbreeding avoidance systems. The purpose of the studies presented herein is to investigate (i) the systems involved in the detection of close relatives and, (ii) the systems entrained to deter motivations for seeking a close relative as a sexual partner. Empirical evidence is presented that indicates disgust is the emotion entrained to regulate sexual attraction toward those categorized as close kin. Furthermore, an adaptationist perspective of disgust is offered that re-evaluates the functional domains of disgust. Using disgust as a measure of sexual aversions, studies were designed to investigate the cues used to assess siblingship by quantitatively matching variation in developmental parameters hypothesized to mediate sibling detection to variation in facial expressions and self-reports of disgust associated with imagining sexual behavior with a sibling. For both males and females, co-residence duration with an older sibling was found to influence sexual aversions. This pattern was not found for co-residence duration with a younger sibling, confirming adaptationist hypotheses regarding the use of co-residence as a cue for younger and older sibling detection. These results explain the sociological patterns of fertility and marriage present in archival data of exceptional populations in China. The development of a sexual aversion between siblings is also found to mediate moral sentiments relating to incest.