#1041 - Dr Debra Lieberman - Why Don’t You Have Sex With Your Sister?

What It Covers

Dr. Debra Lieberman explains the evolutionary psychology behind incest avoidance mechanisms, how humans detect genetic relatives through kinship cues like maternal investment and co-residence duration, and presents a novel theory about crying as social value signaling.

Key Questions Answered

• •Kinship Detection System: Humans identify siblings through two primary cues: observing maternal breastfeeding and investment in newborns, and tracking co-residence duration during childhood dependency years. Each year of shared co-residence increases certainty of genetic relatedness, with exposure from birth producing strongest sexual aversion effects in adulthood.
• •Westermarck Effect Mechanism: Children raised together from early childhood develop sexual aversion regardless of actual genetic relatedness, as demonstrated by Taiwan's minor marriage practice where adopted girls raised alongside sons showed higher divorce rates, more extramarital affairs, and fewer offspring when forced to marry their pseudo-siblings.
• •Genetic Sexual Attraction Paradox: Half-siblings who meet as adults without childhood co-residence lack natural sexual aversion and may experience attraction because they share similar preferences, dispositions, and traits through common genetics, creating compatibility without the disgust response that normally prevents incestuous relationships from forming.
• •Crying as Leverage Signal: Tears communicate social value assessments and cost impositions in relationships. Lower-leveraged individuals use crying to signal when costs become too high or to mark high-value positive events. Women cry more than men because they historically occupied lower-leverage positions in physical formidability and resource control dynamics.
• •Sex Differences in Disgust Response: Women show significantly less variation in sibling incest disgust responses compared to men, consistently rating scenarios at maximum disgust levels. This reflects higher reproductive costs for women, who face nine months gestation and three years lactation, making any mating decision error with reduced offspring viability more evolutionarily costly.