The episode features Debra Lieberman, evolutionary psychologist and editor of Evolution and human behavior, discussing her research on kinship detection, inbreeding avoidance, and emotional signaling.

Lieberman explains how humans and animals identify genetic relatives through kinship cues rather than language, focusing on mechanisms like maternal breastfeeding observation and childhood co-residence duration.

The conversation explores the Westermark effect, natural experiments like Taiwan's minor marriage system, and why adopted siblings raised together develop sexual aversion despite lacking genetic relatedness.

Later topics shift to crying as an evolutionary adaptation, examining why tears function as negotiation tools for lower-leveraged individuals and how they communicate social value in relationships.

How Humans Detect Genetic Relatives Without Language

Evolution engineered sophisticated kinship detection systems because inbreeding produces less healthy offspring with greater genetic mutations, and because Hamilton's inclusive fitness theory shows altruism evolves by being nice to those who share genes by common descent

Animals without language use kinship cues that correlated with genetic relatedness in their evolutionary history, while humans layer language and culture onto these systems imperfectly

"You know who your actual father and mother are... with good certainty, you know who your mom is, who your daddy is, is always an interesting question" - Debra, noting maternal certainty versus paternal uncertainty

Lieberman rejects facial resemblance (phenotype matching) as the primary kinship cue because ancestrally people looked similar and genetic assortment means siblings can look very different while still being closely related

Two Kinship Cues: Maternal Investment and Co-Residence

The first unmistakable kinship cue is observing your mother pregnant, giving birth to, caring for, and breastfeeding a newborn - this operates regardless of actual genetic relatedness

Taiwan's minor marriage system provided natural experiments where families adopted newborn girls to raise alongside sons for future marriage. Mothers often breastfed adopted daughters, triggering the son's kinship detection system despite no genetic relation

Anthropologist Arthur Wolfe documented that these marriages ended more often in divorce with more extramarital affairs

Couples raised together from birth had fewer children, showing sexual aversion despite cultural pressure to marry

The second cue is the Westermark effect, named after Finnish social scientist Edward Westermark, where children raised together throughout childhood develop sexual aversion in adulthood

"Each year of co-residence seems to add a little bit of certainty, or I should say reduces the uncertainty that someone is a genetic relative" - Debra, explaining there's no sharp cutoff but continuous calibration throughout childhood dependence

Starting co-residence from birth is key - individuals who meet later still show the effect but not as strongly as those exposed to siblings from the beginning

Why Incest Porn Exists Despite Strong Disgust Mechanisms

Lieberman hypothesizes that incest porn viewers likely don't have opposite-sex siblings, so they lack the natural sexual aversion that would make the content genuinely disgusting rather than merely taboo

During facial expression research, Lieberman recorded a male participant smiling at "tongue-kiss a sibling" prompt, then frantically checked his survey data to confirm he had no sisters - "it was basically like, imagine having sex with a girl. And he's like, hmm"

Genetic sexual attraction between long-separated relatives occurs because they share many preferences and dispositions (50% of psychology shaped by behavioral genetics) but lack the disgust response that co-residence would have created

"If you remove the incest aversion, if you remove the disgust response, and you're of the sex that... why wouldn't you do this?" - Debra, explaining attraction between separated relatives

Jonathan Haidt's Moral Dumbfounding Experiment

Haidt's Mark and Julie experiment presents consensual adult sibling sex with five condoms, birth control, memory wiping, and no harm to anyone - yet participants still judge it wrong without rational justification

Lieberman challenges the standard interpretation, arguing people don't actually care about harm to Mark and Julie but fear being seen as condoning incest in front of experimenters and other participants

"I think we are very sensitive about moral condemnation... people are very sensitive about being in that room with Jonathan Haidt and the other experimenters to be like, I think incest is okay" - Debra

Sex Differences in Incest Disgust Responses

Lieberman's research found female responses to sibling incest scenarios showed minimal variation, with disgust ratings tightly clustered at ceiling levels, while male responses showed much wider variance

"Females, when selecting a mate, have to consider... nine months of gestation, three years of lactation... getting a baby up and running is easily three, four years ancestrally" - Debra, explaining why women have stronger disgust

Males face far lower opportunity costs for reproduction, so while they shouldn't be pro-sibling incest, they find it "slightly less horrifically objectionable" compared to females

Women generally have lower disgust thresholds than men for most things, with exceptions including changing diapers and having sex during menstruation, where female disgust is lower

Cousin Marriage and Genetic Risk Drop-Off

When Ayla Majluf polled her audience about dating a hot cousin, results were almost exactly 50-50 among 12,000 votes, though her audience skews rationalist and sex-positive

Cousin marriage remains one of the most prevalent forms of marriage worldwide because genetic risks drop dramatically - from 0.5 relatedness for siblings to 0.125 for cousins

Lieberman suspects kinship detection can be tuned up for cousins through observing your mother's sister raising a child in proximity, similar to sibling detection mechanisms

Crying as Evolutionary Negotiation Tool

Lieberman's crying research Evolution and human behavior, prompting her to ask what crying actually does

Crying functions as a tool used by lower-leveraged individuals (less physically formidable, lower socioeconomic status, less attractive, smaller) to negotiate in relationships where they're unlikely to get their way through dominance

"Tears is one way that we use to communicate costs or better yet, the intensity of a particular state... it might be worth your while to stop" - Debra, explaining tears as cost signals

Tears communicate value intensity in both directions - negative (you're imposing costs, please stop) and positive (this unexpected gift/act is extremely valuable to me)

Women cry more than men and kids cry more than adults because they're typically lower-leveraged in physical formidability, making tears a more effective negotiation strategy than anger

Authentic Crying Versus Crocodile Tears

Lieberman proposes an untested way to distinguish real from fake crying: authentic criers hide their tears even in dark movie theaters ("dry up, dry up. Don't let anyone see you crying"), while manipulators display tears prominently

"People who use crocodile tears need them to be on display... don't you see my tears? These are tears because I care" - Debra, contrasting with authentic criers who hide

Psychopaths and dark triad individuals use crocodile tears manipulatively because they understand the social effects without experiencing genuine emotion

Tears serve as a reliable signal because they temporarily incapacitate you by impairing vision, making them costly to produce and therefore harder to fake convincingly

Why Women Cry When Angry

Women tend to tear up when angry, especially in group situations where they're out-leveraged and not being valued, creating a tension between wanting to assert themselves and lacking the physical formidability to back it up

"If I could, I would, but I can't, so I won't. But if I could" - Debra, describing the tension in angry crying where rage wants to burst through but physical reality constrains it

Men don't typically do the "angry cry" in the same way because their rage can burst through more effectively given greater average physical formidability

Crying Alone and Mental Simulation

People cry alone because they're running mental simulations and dramas constantly, similar to laughing alone when thinking about something funny - "we can absolutely simulate what would be my reaction in this particular situation"

Crying at movies or videos (like soldiers reuniting with dogs) demonstrates empathy - our mental simulation system evolved with storytelling and can't fully distinguish between witnessed events and screen content

"Score one for not being a psychopath" - Debra, responding to the host's low crying threshold, noting that psychopaths lack empathy capacity and wouldn't cry at emotional content

Grief, Breakups, and Emotional Recalibration

Lieberman proposes a "hairbrain idea" that post-breakup crying might function as a data dump to recalibrate social value after realizing someone you held in high regard doesn't care about you

She suggests a well-funded lab could capture tears during breakups to measure whether pre and post levels of oxytocin or other attachment-governing chemicals change, testing if crying physically purges social bonding

Crying likely serves multiple functions - some emotions like anger and gratitude calibrate others' behavior, while crying may also recalibrate one's own emotional state

Why Tears Specifically as Emotional Signal

Tears are "front and center" on the face, making them highly visible and difficult to ignore as emotional signals compared to other potential bodily responses

One evolutionary lineage possibility: tears already associated with negative stimuli through irritant response (like onions causing tearing), providing a foundation for emotional co-option

Tears require effort to produce for most people (except psychopaths), making them a relatively reliable authenticity signal that's not cheap to fake

Producing tears temporarily incapacitates vision, functioning as a costly signal that demonstrates genuine vulnerability and inability to defend oneself