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

Overview

This episode dives into the evolutionary psychology behind incest avoidance, exploring how humans and animals detect kinship, the mechanisms that prevent inbreeding, and the moral and cultural dimensions of incest. Dr. Debra Lieberman also discusses the evolutionary purpose of crying and its role in human relationships.

Notable Quotes

- The brain computes one kinship estimate for every person we meet, which determines how nice we should be to them and how avoidant we should be of them sexually. – Dr. Debra Lieberman

- Crying is a tool used by the lower-leveraged to communicate costs or signal value in relationships. – Dr. Debra Lieberman

- If incest disgust is so strong, why is incest porn such a huge category online? – Chris Williamson

🧬 Evolutionary Mechanisms of Incest Avoidance

- Humans have evolved a natural inbreeding avoidance system based on kinship cues experienced during childhood, such as co-residence and maternal care (Dr. Debra Lieberman).

- Animals use non-verbal cues like smell, litter association, or imprinting to detect relatives, as they lack language.

- The Westermarck Effect explains how prolonged co-residence during childhood fosters sexual aversion among siblings, even in the absence of genetic relatedness.

- Cultural practices, like Taiwan's historical minor marriage, reveal how early co-residence can unintentionally trigger incest avoidance mechanisms, leading to higher divorce rates and extramarital affairs.

🧠 Kinship Detection and Altruism

- The same system that detects kin for incest avoidance also drives altruism, aligning with Hamilton's theory of inclusive fitness (Dr. Debra Lieberman).

- Kinship detection relies on cues like maternal investment and co-residence, but it is not foolproof. For example, adopted siblings can develop similar aversions despite no genetic ties.

- Facial resemblance as a kinship cue is debated; while it may spark feelings of kinship, it is not a reliable evolutionary mechanism due to ancestral genetic diversity.

🤔 Moral and Cultural Dimensions of Incest

- Moral disgust toward incest is deeply ingrained, as shown in Jonathan Haidt's moral dumbfounding experiments, where participants struggled to justify their aversion beyond it's just wrong.

- Cultural norms heavily influence incest taboos, but explicit knowledge of kinship (e.g., sperm donor siblings) does not always trigger natural aversion.

- The prevalence of cousin marriage worldwide highlights how aversion decreases with genetic distance, as risks of genetic defects drop significantly from nuclear family to cousins.

😭 The Evolutionary Purpose of Crying

- Tears serve as a social signal, communicating distress or high value to others, particularly in situations where the individual is less powerful or lower-leveraged (Dr. Debra Lieberman).

- Crying can also mark positive events, signaling gratitude or emotional significance.

- Women and children cry more than men and adults, reflecting their historically lower leverage in social interactions.

- Crocodile tears are a manipulative use of crying, often employed by individuals with dark triad traits to feign vulnerability.

📉 Crying and Emotional Recalibration

- Crying may help recalibrate emotional states, particularly during grief or breakups, by processing the loss of social value or attachment.

- Hypotheses suggest tears might chemically expel attachment-related hormones, aiding emotional recovery.

- The act of crying alone could simulate social interactions, reflecting the brain's tendency to prepare for or replay social scenarios.