Why Don’t You Have Sex With Your Sister? - Dr Debra Lieberman

Quick Overview

Humans possess a natural, sophisticated inbreeding avoidance system that develops during childhood through exposure to kinship cues, which serves the dual purpose of preventing mating with close genetic relatives to avoid offspring health issues and promoting altruism towards kin based on inclusive fitness theory.

Key Points: Evolution engineered a sophisticated psychological system to detect close genetic relatives and develop a sexual aversion towards them because mating with kin causes offspring health problems and greater genetic mutations. Kin detection relies on cues correlated with genetic relatedness; in animals, this includes scent or imprinting on a place, while humans primarily use cues like seeing a mother breastfeeding or caring for a newborn, which operates regardless of actual genetic relatedness. The Westermark effect identifies a second cue: co-residence duration, where tracking how long an individual lives under the same roof receiving shared parental investment increases certainty of genetic relatedness, leading to greater altruism and sexual disgust. In natural experiments like Taiwan's minor marriage practice, observed maternal breastfeeding of an unrelated adopted child triggered incest avoidance, although strong cultural norms sometimes overrode this aversion, leading to more divorces and affairs in those unions. Paternal half-siblings raised without these early kinship cues do not develop a natural sexual aversion, explaining why Genetic Sexual Attraction (GSA) might occur when they meet later, as they share similar psychological preferences without the innate disgust trigger. When testing moral dumbfounding scenarios (consensual, safe sibling sex), participants overwhelmingly state it is wrong, though the reason might stem from social conformity/fear of opposing the majority rather than perceived harm to the participants themselves. Females show far less variation in disgust responses toward sibling incest scenarios (scoring near ceiling disgust), while males exhibit wider variation, which is attributed to the much higher reproductive cost females face ancestrally from poor mating decisions.