Armchair Expert with Dax Shepard
Episode: Nancy Segal (on twins)
Date: August 20, 2025
Episode Overview
This engaging episode features Dr. Nancy Segal, evolutionary psychologist, behavioral geneticist, and renowned expert on twins. Dax Shepard and co-host Monica Padman dive deeply into the science and lived experience of twinship, touching on everything from classic twin studies to personal stories, ethical dilemmas, nature versus nurture, and the future of genetic and epigenetic research. The conversation is candid, curious, and filled with both wit and insight.
Key Discussion Points & Insights
Dr. Nancy Segal’s Background & Twin Fascination
[04:41]
- Nancy was born a fraternal twin and her personal experience led to her passion for studying twins.
- Quote (Nancy Segal, 04:57): "I’m a twin and I have a fraternal twin sister and I nudged my way out first... I was only 3lbs 11oz and she was 4lbs 7."
- She explains her academic path—from undergrad at Boston University to the University of Chicago—highlighting her "abnormal psychology" class project that sparked her career-long interest.
Twin Studies: Nature, Nurture, and Extreme Environments
[06:23], [09:00]
- Nancy emphasizes that while firstborn twins tend to be physically healthier at birth, long-term differences are not marked in twins raised together.
- However, twins raised apart, especially in drastically distinct environments, provide strong evidence of both genetic predisposition and the overwhelming power of environment in extreme cases.
- She shares the story of the "Accidental Brothers," two sets of identical twins switched at birth in Colombia, which provided pure data on nature versus nurture.
Quote (Nancy Segal, 10:12): "For someone like me, a fraternal twin... the findings initially shocked me, but... in extreme nature, environment overwhelmed genetics."
Historical Context of Twin and Genetic Research
[10:39], [12:00]
- Dax and Nancy discuss the evolution of genetic research, from early behaviorism’s focus on nurture to post-WWII skepticism around genetics, all the way to the renaissance of twin studies in later decades.
- Josef Mengele’s unethical experiments tainted twin research for decades; acceptance returned in the 1980s when missing environmental explanations required genetic answers.
The Jim Twins: Iconic Case of Twins Separated at Birth
[15:55]
- The Jim Twins—separated at birth, reunited at age 39—shared astonishing similarities (careers, hobbies, relationships, quirks), fueling public intrigue and challenging the assumption of coincidence.
- Quote (Dax Shepard, 16:13): "But the craziest is they both married Linda first, divorced, married Betty, and then married Betty..."
- Nancy argues that such similarities are often rooted in genetic predispositions rather than mere coincidence.
Genetics, Environment, and Personality
[18:25], [20:27]
- Deep dive into fraternal vs. identical twins: identical twins come from one split egg, sharing almost all genes; fraternals come from separate eggs, just like regular siblings.
- Nancy busts the myth that the shared womb makes identicals more similar—if anything, twin pregnancy introduces more differences.
Kinship Theory and Inclusive Fitness
[21:30]
- Nancy explains Hamilton’s kinship theory: we’re more altruistic toward people we share genes with, influencing social behavior, family closeness, and even investment in nieces/nephews.
- Dax ties this to the "selfish gene" idea of Dawkins.
Doppelgangers vs. Identical Twins
[24:10]
- Nancy discusses her research into doppelgangers—unrelated people who look similar—and finds they are not personality matches, debunking the idea that outward resemblance causes behavioral similarity.
- Quote (Nancy Segal, 25:17): "No [doppelgangers] are not [similar in personality]. They're not close... unlike identical twins, who become closer over time."
Twins and Individuality; Social Perceptions
[26:10], [34:10]
- Discussion of how twins challenge Western ideas of individuality, and how both individualistic and collectivist cultures remain fascinated with twins.
- Monica muses that fraternals strive more for individuality, but Nancy counters that identical twins have to work harder to assert their uniqueness.
Parenting, Genetics, and the Myth of Nurture
[35:39]
- Parents of multiples (especially fraternals) become natural geneticists, quickly realizing environmental control is limited.
- Nancy suggests that parents of one child often over-emphasize nurture, given the illusion of control.
The Balance of Nature and Nurture
[38:02]
- In classic studies (like the famous 1960s triplets in “Three Identical Strangers”), similarities among separated twins occurred even when not sought out.
- Social attitudes, religiosity, and preferences tend to be more genetically driven than previously believed—though environment matters, especially in early childhood.
Unethical Twin Studies & Separated Twins
[40:11]
- Nancy broke down the notorious Louise Wise Services case and the ethics (or lack thereof) of “deliberately divided” twins in research.
- She critiques studies that excluded fraternal twins as controls, highlighting methodological flaws.
The Evolving Value of Twin Research in the Genomic Era
[42:01], [44:00]
- With the human genome mapped, the role of twins has shifted: today, their main value is to illuminate the impact of the environment and epigenetics (gene expression changes due to environment).
- Nancy elucidates how twins are still a model for studying broader human behavior, especially noting discordant cases (e.g., one twin develops Alzheimer’s or is transgender).
Epigenetics and Inherited Trauma
[47:55]
- Discussion on Lamarckian (acquired) inheritance, citing evidence of trauma’s epigenetic transfer in Holocaust survivors' children, though such changes may fade across generations.
Famous (and Infamous) Twins: The Mary-Kate and Ashley Debate
[49:07]
- Nancy confidently asserts that Mary-Kate and Ashley Olsen are almost certainly identical twins, despite their public statements—highlighting the ambiguity when DNA testing isn’t done.
Twin Marriage and Mate Selection
[51:00], [52:25]
- Nancy delves into identical twins marrying identical twins, their children’s genetic relationships (full siblings), and how even within identical twin pairs, partner selection isn’t random.
- She previews research showing identical twins invest more in their co-twin’s children than fraternals.
The Unique Social World of Twins
[56:56], [58:46]
- Nancy shares moving reunion stories (Chinese twins separated by adoption, reunited from Norway and Sacramento); even with language barriers, their play and connection was instant.
- Discusses “twin language” (idiolects), twin self-esteem, and the unique type of mutual acceptance twins share.
Loss, Grief, and Twin Bonds
[63:48]
- Nancy’s research finds grief is more intense for identicals who lose a twin, partly because “you’re always looking for her,” and it's hard to find that level of connection elsewhere.
Cloning and the Future
[68:09], [69:18]
- As speculative reproductive technologies advance, Dax, Monica, and Nancy debate the ethics and psychology of cloning oneself, with Nancy using the twin model to anticipate possible outcomes.
Notable Quotes & Memorable Moments
-
On Genes and Responsibility:
“Keep in mind that genes just predispose—they don’t determine.”
—Nancy Segal, [04:35] -
On Surprising Twin Similarities:
“How do you explain two identical twins raised apart... who both washed their hands before and after using the bathroom?”
—Nancy Segal, [17:12] -
On Twin Language:
“People call it twin language, but it’s really not a language. ...It’s a system of communications—words, gestures, phrases that twins evolve because they’re so close.”
—Nancy Segal, [57:43] -
On Grief and Loss Among Twins:
“I do find grief intensity higher among the identicals than the fraternal. ...They wish they had gone first.”
—Nancy Segal, [63:48] -
On Parenting and Nurture:
“You don’t bring up your child, your child brings you up. Because parents find that what works with one kid doesn’t always work with the other kid.”
—Nancy Segal, [35:39] -
On Identity and Replication:
“Why is one identical twin attracted to someone and her twin is not? I find that fascinating.”
—Nancy Segal, [49:56]
Informative Segment Timestamps
| Timestamp | Segment | Topic/Note | |------------|-------------|------------------------------------------------------| | 04:41 | Start of interview | Nancy’s background, personal twin story | | 06:23 | Twin birth order | Health, environment, and outcomes | | 09:00 | Extreme environments | Colombian accidental twins, nature vs. nurture | | 12:00-13:00| Historical context| Genetics post-WWII, impact of Mengele, women’s rights| | 15:55 | Jim Twins | Famous case, behavior overlap, significance | | 20:27-21:27| Fraternal vs identical | Genetic similarity, the womb's mixed effects | | 21:30 | Kinship theory| Basis of altruism, inclusive fitness | | 24:10 | Doppelgangers| Research findings, personality and resemblance | | 35:39 | Parenting | “Geneticists vs. environmentalists” among parents | | 38:02 | Confirmation bias | Twin studies, data approaches, “Three Identical Strangers”| | 40:11 | Ethics/History| Separated twin studies, Louise Wise Services [see also 41:59 for 'Twinning Reaction' doc] | | 42:01 | Genomics & twins| The evolving value of twin research | | 47:55 | Epigenetics/trauma| Inheritance of trauma, Holocaust survivors | | 49:07 | Olsen twins | Identity debate, handedness among twins | | 51:00-52:25| Twin marriage | Genetic implications, parenting, mate selection | | 56:56 | Twin reunions| Emotional stories, instant connection | | 58:46 | Self-esteem | Social advantages of twin partnership | | 63:48 | Loss/grief | Impact on surviving twins, unique bereavement | | 68:09 | Cloning | Philosophical and practical considerations |
Episode Tone & Style
- Warm, candid, curious: Dax’s anthropological interest pairs well with Nancy’s academic approach but real-life warmth.
- Anecdotal but evidence-driven: Blending memorable stories (the Jim Twins, giggle twins, separated Chinese twins) with data and theory.
- Accessible: Complex genetics, evolution, and psychology are explained plainly for listeners without prior background.
Final Thoughts
This episode is a gold mine for anyone fascinated by what makes us who we are: the tug-of-war between genetics and environment, individuality and kinship, chance and fate. Dr. Segal’s work challenges listeners to rethink the bounds of personal identity, choice, and connection—and offers a compassionate, scientific look at the complexity of being human, together and apart.
Recommended further reading:
- “Entwined Lives,” “Born Together, Reared Apart,” “Accidental Brothers,” and “Deliberately Divided” by Nancy Segal.
- "Three Identical Strangers" & "The Twinning Reaction" documentaries.
(For fact-check clarifications on handedness, spatial/verbal gender differences, and the Olsen Twins’ DNA status, see Monica’s fact check at [109:17])