Modern Wisdom #1041 - Dr Debra Lieberman - Why Don’t You Have Sex With Your Sister?
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Jan 3, 2026 
Dr. Debra Lieberman, an evolutionary psychologist, dives into the fascinating mechanics behind why humans are hardwired to avoid incest, sharing insights from her extensive research. She discusses how animals detect kin through various cues and how this translates to human behavior. The moral complexities of incest, genetic risks of cousin relationships, and the unique impact of childhood co-residence are explored. Additionally, they touch on the intriguing social signaling of tears, highlighting their role in communication and social dynamics.
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Evolved Kin Detection Guides Sex And Altruism
- Humans evolved a kin-detection system that reduces sexual attraction to close relatives and promotes altruism.
- The brain uses childhood cues to estimate relatedness and adjusts both sexual aversion and helping behavior accordingly.
Kinship Detected By Correlated Cues
- Animals and humans use correlated cues (smell, imprinting, co-residence) to detect relatives without language.
- Humans map language and culture onto these cues, but the underlying mechanisms are similar across species.
Breastfeeding And Co-Residence Build Aversion
- Two powerful sibling cues are maternal caregiving (breastfeeding) and duration of childhood co-residence.
- These cues, known as maternal investment and the Westermark effect, raise kin certainty and sexual aversion.