EconTalk

Nature, Nurture, and Identical Twins (with David Bessis)

January 19, 2026

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  • The widely cited studies of twins reared apart, which suggest genetics are destiny regarding traits like IQ, suffer from significant methodological flaws, including small sample sizes and a lack of necessary control groups, undermining their strong claims about heritability. 
  • Behavioral geneticist Eric Turkheimer's third law posits that a large portion of human variability in traits is accounted for by unique, idiosyncratic experiences that are neither genetic nor attributable to shared family environment. 
  • Mathematician David Bessis argues that exceptional achievement, even in highly 'G-loaded' fields like mathematics, results from a unique, non-innate journey involving metacognitive approach, patience, and persistence, rather than being solely predetermined by genetics. 

Segments

Guest’s View on Genius
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(00:00:48)
  • Key Takeaway: Great mathematicians achieve greatness through powerful, learnable methods, dogged focus, and hard work, contradicting the view that they are uniquely born geniuses.
  • Summary: David Bessis argues that mathematical greatness is not innate but developed through a powerful, learnable approach. He cites figures like Descartes and Einstein who emphasized method and hard work over unique genius. This perspective directly challenges the idea that mathematical talent is predetermined by genetics.
Provocation by Twin Studies
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(00:01:59)
  • Key Takeaway: The claim that twins separated at birth show IQ results nearly identical to retesting the same person suggests extreme genetic determinism, which Bessis finds disturbing.
  • Summary: The guest was disturbed by the popular claim derived from twins-reared-apart studies: that separated identical twins show IQ divergence no greater than two tests on the same individual. This finding implies that nature overwhelmingly dictates destiny, contradicting Bessis’s belief in the power of learned approaches.
Heritability vs. Determinism
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(00:03:29)
  • Key Takeaway: Genes matter, but the debate hinges on whether they are part of the determination or the entire determination of outcomes.
  • Summary: Bessis clarifies he does not believe in the extremes of genetic determinism or the blank slate; genes clearly matter, evidenced by genetic defects affecting cognition. However, the core issue is the degree of influence, as even weak correlations, like those involving brain size, suggest genes are only part of the picture.
Critique of Twin Study Popularity
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(00:04:59)
  • Key Takeaway: The foundational twin studies, widely publicized, date from the 1950s through the 1980s and are often presented without necessary scientific nuance.
  • Summary: The studies underpinning genetic determinism claims were conducted decades ago and have been widely publicized, notably via social media posts by respected figures like Paul Graham. If these studies were true, they would imply deep genetic determination, which Bessis finds personally and scientifically problematic.
The Thurston Genius Example
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(00:09:17)
  • Key Takeaway: The unique, idiosyncratic journey of mathematician Bill Thurston, overcoming a childhood squint through persistent training, led to his 4D/5D visualization abilities and Field Medal.
  • Summary: Bill Thurston, born with a squint, developed extraordinary 3D and higher-dimensional visualization skills through intensive, loving training by his mother and daily practice. This unique journey propelled him from potentially being in the top 1% to becoming a world-renowned geometer, illustrating the power of idiosyncratic experience.
Interpreting Heritability Percentages
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(00:12:11)
  • Key Takeaway: A 50% heritability estimate still leaves substantial room for individual choice and environmental factors to shape outcomes like IQ.
  • Summary: Visualizations show that a 50% heritability for IQ implies a trend but not a strong correlation, meaning individuals with average polygenic scores still have a chance to reach the top 1%. The remaining non-genetic factors include idiosyncratic experiences that are not easily categorized by simple socioeconomic criteria.
Turkheimer’s Three Laws
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(00:16:48)
  • Key Takeaway: Turkheimer’s third law states that a huge amount of human variability comes from factors neither genetic nor shared family environment, encompassing chance encounters and personal experiences.
  • Summary: The three laws of behavioral genetics clarify the debate: 1) Everything is heritable; 2) Family environment usually matters less than the genome; 3) A large portion of variability is neither genetic nor family-based. This third law emphasizes the critical role of unique life events, friends, and personal mental processes.
Nuance in Sibling Differences
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(00:19:51)
  • Key Takeaway: The low correlation between siblings (compared to identical twins raised together) suggests that parental influence is not uniform but highly dependent on the unique dynamics between parent and each specific child.
  • Summary: While identical twins raised together show high correlation, siblings raised together show weaker correlation, which can be interpreted as evidence against the importance of family environment. However, this difference may reflect that each child experiences a unique, non-linear developmental path within the same family structure.
Bouchard Study Design Flaws
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(00:33:26)
  • Key Takeaway: The Bouchard study, intended as a perfect natural experiment, failed to control for shared prenatal environment and the non-random, potentially negative circumstances leading to twin separation.
  • Summary: Twins share nine months of prenatal environment, which can bias IQ correlations, and separation often occurred due to unusual, potentially negative family circumstances. The proper de-biasing technique involves comparing identical twins reared apart to fraternal twins reared apart, a control group the Bouchard team failed to publish data for.
Omission of Control Data
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(00:37:35)
  • Key Takeaway: The Bouchard team’s decision to omit the fraternal twin control group data from their 1990 Science publication, citing space constraints, is an unacceptable lack of transparency for a study with major societal implications.
  • Summary: The researchers claimed space constraints prevented publishing the fraternal twin control data, which is essential for calculating heritability estimates correctly using the Falconer method. This omission is highly suspect because the control data would have provided the crucial comparison needed to validate the identical twin findings.
Importance of Human Agency
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(00:52:34)
  • Key Takeaway: The intense public passion surrounding the nature versus nurture debate reflects a deep human need to understand and claim agency over one’s own fate and identity.
  • Summary: People care deeply about the nature vs. nurture split because it strikes at the core of self-perception and control over one’s destiny. The desire to believe one’s journey is self-directed, as Bessis found regarding his math talent, is a powerful psychological driver in this controversy.