Modern Wisdom with Chris Williamson

Episode #1041 - Dr. Debra Lieberman
Title: Why Don’t You Have Sex With Your Sister?
Date: January 3, 2026
Guest: Dr. Debra Lieberman, evolutionary psychologist and kinship expert

Overview

In this engaging episode, Chris Williamson sits down with Dr. Debra Lieberman to explore the evolutionary roots of incest avoidance, discuss the mechanisms humans (and other animals) use to identify kin, and dive into the psychology behind other emotions such as disgust, gratitude, and crying. The discussion weaves between taboo topics and evolutionary insights, shedding light on why certain aversions exist, how they’re formed, and what they mean for modern behavior and morality.

Key Topics & Insights

1. The Evolution of Incest Avoidance

[00:00–07:14]

• Why Don't We Feel Sexual Attraction Toward Our Siblings?
  Dr. Lieberman explains that humans have evolved sophisticated systems to avoid inbreeding because of its negative consequences on offspring health.

  • “Evolution engineered into our psychology a very sophisticated system to allow us to detect relatives, close genetic relatives, and develop a sexual aversion towards them.” — Dr. Lieberman [00:48]

• Kin Detection in Animals and Humans
  Animals use non-verbal cues (littermates, smell, imprinting on location or markings) to detect kin in the absence of language.

  • “Humans have language, so language and culture map very nicely onto these systems, but they’re imperfect.” — Dr. Lieberman [01:43]

• The Dual Function of Kin Detection
  Kinship estimation is used both to promote altruism (cooperation) and to avoid sexual relationships with close genetic relatives.

  • “The same kin detection system is operating for evaluating someone as a sexual partner and as someone to be altruistic to.” — Dr. Lieberman [06:54]

Memorable Quote:

• “How close my heart should be and how far my genitals should be from this person.” — Chris Williamson [07:50]

2. Psychological Cues to Kinship

[07:51–13:01]

• Key Cues:

  • Maternal Care/Breastfeeding: Seeing your mother care for or breastfeed another baby indicates a sibling, regardless of genetic relatedness.
    • “That's an unmistakable cue that their kid… being cared for by my mom is my sibling. And that operates regardless of actual genetic relatedness.” — Dr. Lieberman [08:19]
  • Co-residence Duration (Westermarck Effect): The longer two children live together during early life, the stronger the incest aversion.
    • “The longer the duration of co-residence during early childhood, the more certain you are that an individual is a genetic relative.” — Dr. Lieberman [12:35]

• Natural Experiments:
  Examples such as Taiwan's “minor marriage” system demonstrated that cultural constructs can override—but not erase—evolved sexual aversion; such couples had higher divorce and infidelity rates.

  • “When they were raised together from birth… those marriages tended to end more often in divorce and there were more extramarital affairs.” — Dr. Lieberman [11:14]

3. Sex Differences and Disgust

[23:09–28:13]

• Greater Female Disgust:
  Studies show women consistently rate incest scenarios at the highest levels of disgust compared to men, reflecting the higher evolutionary cost of poor mate choice for women.

  • “Females… were all at ceiling. The amount of variation that I found in female responses… was like basically touching… all very squeezed right up at ceiling. Males… the variance structure was quite wide, which I found humorous.” — Dr. Lieberman [23:30]
  • “Any bad decision in terms of investing… into an offspring with a reduced chance of survival would have been heavily selected against.” — Dr. Lieberman [24:24]

• Disgust Thresholds:
  Women tend to have a lower disgust threshold for most things, though with exceptions (e.g., changing diapers, sex during menstruation).

  • “There are a couple things that flipped… females were like ‘that’s fine’.” — Dr. Lieberman [25:32]

4. Modern Manifestations & Outliers

[28:13–36:17]

• Only Children & Incest Aversion:
  Those without siblings (e.g., only children) show a muted disgust response to hypothetical incest scenarios—they simply lack the cues that would’ve triggered this aversion system.

  • “Without that natural aversion, you don’t have such a strong reaction.” — Dr. Lieberman [30:11]

• Incest Porn & Taboo-Fetishization
  The popularity of incest-themed pornography is discussed, with both acknowledging it’s unlikely consumed by people with strong sibling bonds. The draw may partly be the thrill of violating social norms when no genuine disgust response exists.

  • “The hypothesis would be that a guy with six sisters is not the person who's watching this.” — Dr. Lieberman [32:50]

• Cousin Marriage Prevalence:
  Aversion drops off rapidly beyond the nuclear family; cousin marriages are (and have been) common in various cultures.

  • “Cousin marriage is still… one of the more prevalent forms of marriage worldwide.” — Dr. Lieberman [34:48]

Notable Exchange:

• Chris: “That would be such a good study… Does the likelihood of watching incest porn go down based on the number of opposite sex siblings?” [32:59]
• Debra: “I’m really hoping someone in your audience will now undertake that.” [33:18]

5. Evolutionary Story Behind Crying

[38:57–66:28]

• Crying as Social Signaling:
  Tears evolved as a signal primarily in situations where someone lacks leverage or power in a social interaction, prompting others to stop imposing costs or to offer help.

  • “Tears is one way that we use to communicate costs or, better yet, the intensity of a particular state… a tool used by the lower leveraged to get other people to stop imposing costs or to restart the delivery of benefits.” — Dr. Lieberman [41:04]

• Crocodile Tears vs. Genuine Crying:
  “Crocodile tears” (manipulative crying) tend to be performed openly as a strategic display, unlike genuine tears which we often try to conceal.

  • “People who use crocodile tears need them to be on display.” — Dr. Lieberman [44:26]

• Sex Differences in Crying:
  Women and children cry more than men and adults, reflecting their relative lack of power or leverage, on average, in social situations.

  • “Women cry more than men, kids cry more than adults.” — Dr. Lieberman [43:54]

• Crying Alone or at Media:
  Dr. Lieberman suggests even solitary crying functions as a simulation:

  • “I think there’s simulations all the time… When we think about things, we can absolutely simulate what would be my reaction in this particular situation.” — Dr. Lieberman [53:23]

6. Group Dynamics & Formidability

[47:34–51:37]

• Nonverbal Status Games:
  Chris reflects on male group dynamics—nonverbal cues, head of the table, who commands silence—interpreted as real-time calculations of hierarchy and formidability.
  • “If you've got some big dude in the room and he starts speaking… what is the likelihood that some or all people shut up?” — Chris Williamson [49:05]

7. Crying—Function and Future Research

[61:15–66:28]

• Ruminations on Tears as Costly Signals:
  The act of crying (with actual tears) has a cost—it impairs vision and exposes vulnerability, making it hard to fake and therefore a reliable signal of genuine distress or need.

  • “It's front and center… Something that is readily available… This is a negative stimulus.” — Dr. Lieberman [59:20]

• Speculation on Biochemical Functions:
  Dr. Lieberman ponders whether crying serves a physiological “data-dump” role during emotional recalibration, particularly following social loss.

  • “Is it kind of being dumped out through the tears to… reestablish [emotional equilibrium]?” — Dr. Lieberman [64:54]

Notable Quotes & Timestamps

• On kin detection:

  • “We seek, and we're always looking for people who might value us. So looking for overlap in any dimension is something we do naturally to form friendships.” — Dr. Lieberman [06:12]

• On incest aversion:

  • “You telling me that a person is my sibling… that's not really going to do very much to me, that explicit information…” — Dr. Lieberman [16:22]

• On cousin attraction:

  • “As soon as you start to drop off of nuclear family members, the biological deleterious and harmful effects of mating with close relatives drops off pretty fast.” — Dr. Lieberman [35:13]

• On crying as costly signal:

  • “You really need your eyes… and if you choose to have water pour out of them, you have temporarily incapacitated yourself. And if you… a costly signal add on.” — Chris Williamson [60:24]

• On “crocodile tears”:

  • “People who are trying to use their tears for more manipulative purposes will cry and be like, ‘Don't you see my tears? These are tears because I care.’” — Dr. Lieberman [44:26]

• On the dual function of crying:

  • “How much of crying is about changing other people's behavior and how much is about recalibrating our own emotional state?” — Chris Williamson [65:20]

References & Further Resources

• Dr. Debra Lieberman
  • Research, articles, and resources are available at the Center for Evolutionary Psychology.
    • “The Center for Evolutionary Psychology is where I would direct everyone.” — Dr. Lieberman [66:59]
  • Upcoming textbook and a business (Mediabyte) to facilitate access to academic journal articles.

Episode Highlights & Memorable Moments

• Chris’s blunt summary of kin detection as:
  “How close my heart should be and how far my genitals should be from this person.” [07:50]

• Dr. Lieberman’s candid reflections on researching incest aversion and the discomfort of discussing such topics even with fellow scientists.

  • “It was very disturbing talking about these things with your advisors.” [28:13]

• Discussing the popularity of incest porn and the hypothesis that only-children or those lacking strong kin cues might be more likely to consume it.

  • “I don’t know. The hypothesis would be that a guy with six sisters is not the person who's watching this.” — Dr. Lieberman [32:50]

• Explorations into moral dumbfounding around incest:

  • “I actually think that the reason why people say that it's wrong is… not because of Julie and Mark, I don't think people care about Julie and Mark. I think people care about being in that room with Jonathan Haidt and the other experimenters…” — Dr. Lieberman [21:40]

• Dr. Lieberman’s inventive speculation on whether we “tear out” emotional attachments after breakups, suggesting a novel direction for future research.

  • “If you could possibly capture people's tears as they are going through a breakup to see whether… whatever it is that kind of governs social value and attachment… is it kind of being dumped out through the tears...” [64:54]

Final Thoughts

Chris and Debra’s conversation deftly navigates the boundaries between biology, emotion, and taboo. Listeners walk away understanding that disgust and aversion toward incest—far from arbitrary—are the result of deeply embedded systems shaped by evolution to preserve the health of offspring and promote social cohesion. Emotional signals like crying turn out to be tools for both social communication and personal psychological recalibration. The episode balances humor, depth, and openness, making for a memorable exploration of some of human nature’s most profound operating systems.

To learn more about Dr. Debra Lieberman's work

• Visit the Center for Evolutionary Psychology
• Watch for her upcoming textbook (2026/2027)
• Stay tuned for Mediabyte: single-article access to academic journals

For the full conversation, listen to Modern Wisdom, Episode #1041.