What Twins Separated At Birth Teach Us About Human Genetics - Nancy Segal

Nancy Segal is a professor at California State University, an author and one of the most prominent twin-study researchers on the planet. What makes you the person you are? Is it your genetics or your environment? When twins are either accidentally or purposefully separated at birth, it presents a fascinating view into this debate, shedding light on the influence of both traits and environments on the outcomes we get in life. Expect to learn the craziest similarities between twins who have been raised apart, how it's possible to have a pair of twins born at the same time to two different fathers, what happens when twins meet for the first time, what you can learn from triplets that you can’t learn from twins, just how big of an impact genes and environment are on the traits we see in people, how important of a role chance plays in our lives and much more... Sponsors: Get the Whoop 4.0 for free and get your first month for free at http://join.whoop.com/modernwisdom (discount automatically applied) Get a Free Sample Pack of all LMNT Flavours with your first box at https://www.drinklmnt.com/modernwisdom (automatically applied at checkout) Get 5 Free Travel Packs, Free Liquid Vitamin D and more from AG1 at https://drinkag1.com/modernwisdom (discount automatically applied) Extra Stuff: Get my free Reading List of 100 books to read before you die → https://chriswillx.com/books/ To support me on Patreon (thank you): https://www.patreon.com/modernwisdom #twins #genetics #behaviour - 00:00 How Evolutionary Psychology Explains Twins 04:51 Unique Connections Between Identical Twins 08:22 The Rare Occurrence of ‘Half Twins’ 16:22 How Do Twins Know They’re Twins? 21:02 When Separated Twins Are Both Gay 25:00 Craziest Similarities Between Separated Twins 35:19 Do Twins Also Share Illnesses? 43:25 The Ethics of Separating Twins at Birth 48:31 Why Do So Many People Have an Issue with Behavioural Genetics? 52:57 How Much Can Rearing Change a Child? 1:04:14 The Family Dynamic of Bi-Racial Twins 1:07:03 Common Criticisms of Twin Studies 1:11:34 ‘Tell Me Who I Am’ on Netflix 1:14:10 Where to Find Nancy - Get access to every episode 10 hours before YouTube by subscribing for free on Spotify - https://spoti.fi/2LSimPn or Apple Podcasts - https://apple.co/2MNqIgw Get my free Reading List of 100 life-changing books here - https://chriswillx.com/books/ - Get in touch in the comments below or head to... Instagram: https://www.instagram.com/chriswillx Twitter: https://www.twitter.com/chriswillx Email: https://chriswillx.com/contact/

Chris WilliamsonhostNancy Segalguest

Aug 13, 20231h 16mWatch on YouTube ↗

WHAT IT’S REALLY ABOUT

Separated twins reveal genetics’ power over personality, bonds, and destiny

Nancy Segal, a developmental psychologist and twin researcher, explains how twin studies illuminate the interplay of genetics, environment, and evolution in shaping human behavior. She describes why identical twins tend to be more cooperative, emotionally bonded, and behaviorally similar than fraternals, even when raised apart, and how this supports kin selection and behavioral genetics findings. Segal shares striking case studies—from Holocaust-surviving twins and politically opposite reared-apart twins to the Jim twins and accidental incest—showing deep genetic influences alongside environmental variation. She also discusses ethical issues around deliberately separating twins, the impact of genetic understanding on parenting, and a landmark citizenship case involving gay fathers and twin sons conceived via assisted reproduction.

IDEAS WORTH REMEMBERING

5 ideas

Identical twins’ cooperation and closeness are rooted in shared genetics, not just appearance or treatment.

Segal’s puzzle experiments and observational work show identical twins intuitively coordinate and cooperate more than fraternals, reflecting similar temperaments and information processing rather than social labeling alone.

The identical twin bond can be stronger than any other human relationship.

Grief research indicates identical twins mourn a co-twin more intensely than parents, siblings, or even children, aligning with kin selection theory that predicts deeper investment in closer genetic relatives.

Twins raised apart still show remarkable, often eerie, similarities across trivial and major life domains.

Cases like the Jim twins and Oscar and Jack demonstrate shared habits, preferences, and idiosyncrasies despite different upbringings, suggesting people gravitate toward environments and choices compatible with their genetic dispositions.

Genetic influence on many traits increases with age as people select their own environments.

Heritability of IQ, religiosity, and sports participation rises over the lifespan because, once free from parental control, individuals choose activities and contexts that fit their innate tendencies, amplifying genetic effects.

Understanding genetics should inform, not paralyze, personal responsibility and parenting.

Segal argues genes are propensities, not prescriptions: knowing family risks (e.g., alcoholism) or a child’s temperament allows people to shape environments and choices more wisely, rather than succumbing to fatalism.

WORDS WORTH SAVING

5 quotes

Behavior is not built on appearance. Behavior is built on the brain.

— Nancy Segal

I would assert that the identical twin bond is closer than any other relationship.

— Nancy Segal

It’s not like behavior is coming out of nowhere; it’s coming out of somewhere.

— Nancy Segal

Genetics does not say it’s completely genetic… your genes don’t tell you what to do. You’re the one in command.

— Nancy Segal

Parents do not mold you into what you are. Good parenting is being sensitive to the child and helping them become more of who they are.

— Nancy Segal

Behavioral genetics and evolutionary psychology in twin researchDifferences between identical and fraternal twins in behavior and bondingTwins raised apart and extreme similarity case studiesKinship genetics, grief, and the emotional intensity of the twin bondEthics of separating twins in adoption and researchGenetic and environmental influences on traits (IQ, mental health, BMI, etc.)Parenting, behavioral genetics, and implications for child-rearing

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