Flawed Twin Studies Undermine Genetic Determinism Claims

The seductive simplicity of genetic determinism, as popularized by certain interpretations of twin studies, fundamentally misrepresents the complex interplay of nature, nurture, and individual experience. This conversation with mathematician David Bessis reveals how the very studies often cited to prove genetic destiny are riddled with methodological flaws and misinterpretations. The hidden consequence of uncritically accepting these studies is a dangerous oversimplification of human potential, leading to fatalistic views on intelligence, talent, and personal agency. Individuals seeking to understand the true drivers of achievement, particularly in cognitively demanding fields, will gain a crucial advantage by recognizing the limitations of genetic determinism and embracing the power of idiosyncratic journeys and deliberate practice.

The Flawed Foundation of Genetic Destiny: Unpacking the Twins Reared Apart Myth

The notion that genetics dictates our destiny, particularly concerning intelligence, is a powerful and persistent narrative. It finds a seemingly robust empirical basis in the famous studies of identical twins reared apart, often presented as the ultimate natural experiment separating nature from nurture. The compelling, albeit oversimplified, takeaway is that even when raised in vastly different environments, identical twins exhibit remarkably similar IQs, suggesting their genes are the primary, if not sole, determinant of their cognitive abilities. This narrative, amplified across social media and popular discourse, suggests a hardwired destiny, leaving little room for individual effort or environmental influence. However, mathematician David Bessis, in his conversation with Russ Roberts, meticulously dismantles this simplistic view, exposing the profound methodological shortcomings and interpretive biases that undermine the very foundation of genetic determinism.

The allure of the "twins reared apart" studies is undeniable. They promise a clean, decisive answer to one of humanity's oldest questions. As Thomas Bouchard, a key researcher in this field, described his initial fascination, it was the prospect of a "beautiful idea" that would provide a "complete precise and definite answer to the question of nature versus nurture." This seductive promise, however, masks a reality far more complex and less definitive. Bessis argues that the very experiment, as popularly understood, "just doesn't exist in real life." The core issue lies not just in the rarity of twins being separated at birth and subsequently studied, but in the inherent biases and assumptions embedded within these studies.

One of the most significant overlooked factors is the shared prenatal environment. Identical twins spend nine months together in the womb, a critical period for development, particularly for cognitive functions. This shared biological incubator introduces a baseline correlation between the twins that predates their separation. Furthermore, the notion of being "separated at birth" is often an oversimplification. Many studies involved twins separated at around five months, still a significant period of shared development. More critically, the circumstances leading to adoption and separation were rarely random. These were often instances involving social services, potentially indicating dysfunctional family environments or specific joint events that influenced the separation. This lack of true randomization means that the twins might share not only genetics but also certain adverse environmental factors or predispositions that could influence their outcomes, thus conflating genetic and environmental influences.

"The fact that it's special is actually very important in the story because you try to extrapolate general laws about human cognition that apply to billions of people and you try to learn that from 50 something pairs of people in a very unusual circumstance and just from that there's an issue."

-- David Bessis

The scientific rigor expected in modern research is further called into question by the handling of control groups. The gold standard for debiasing such studies, as outlined by geneticists like Falconer, involves comparing identical twins reared apart with fraternal twins reared apart. Fraternal twins share only 50% of their DNA, providing a crucial baseline for distinguishing genetic influence from shared environment. Thomas Bouchard's seminal 1990 Science paper, which presented findings on identical twins, notably claimed to omit its fraternal twin control group data due to "space constraints" and the sample being "too small." This explanation, as Bessis points out, is highly implausible and raises significant red flags. The omission of such critical data, especially when it could have provided a more nuanced understanding of heritability, suggests a reluctance to present findings that might complicate the deterministic narrative.

"The fact that it's special is actually very important in the story because you try to extrapolate general laws about human cognition that apply to billions of people and you try to learn that from 50 something pairs of people in a very unusual circumstance and just from that there's an issue."

-- David Bessis

This lack of transparency and methodological rigor is particularly troubling given the profound implications of these studies. The narrative of genetic determinism, when accepted uncritically, can lead to a fatalistic outlook on personal development and societal interventions. It can foster a belief that our potential is fixed at birth, diminishing the perceived value of education, parenting, and individual effort. This is precisely where Eric Turkheimer's three laws of behavior genetics offer a more balanced perspective. The first law acknowledges that heritability plays a role. The second suggests that the family environment matters less than genetics, a sobering thought for those who believe in the blank slate. However, the crucial third law posits that a "huge amount of variability" stems from neither genetics nor family environment, but from unique individual experiences. This includes chance encounters, personal intellectual journeys, and the development of metacognitive skills--the very elements that Bessis emphasizes in his own work on mathematical talent.

The story of Bill Fursten, a mathematician who overcame a squint to develop extraordinary geometric abilities by practicing visualization and thinking in higher dimensions, exemplifies Turkheimer's third law. Fursten's journey, while perhaps rare in its scale, highlights how persistent, idiosyncratic effort can lead to exceptional abilities, even in fields presumed to be heavily genetically influenced. This contrasts sharply with the deterministic view, where such achievements are attributed solely to innate genius. The implication is that while genetics might set a baseline or influence predispositions, it is the unique, often effortful, journey that truly shapes an individual's capabilities and achievements. By obscuring the nuances of these studies and promoting a deterministic interpretation, the popular understanding of genetic influence risks overlooking the profound impact of personal agency and the vast, often unpredictable, landscape of individual experience.

• Immediate Action: Re-evaluate "Nature vs. Nurture" Narratives: Critically examine claims about genetic determinism, especially those citing twin studies. Seek out analyses that discuss methodological limitations and biases.
• Immediate Action: Embrace Turkheimer's Third Law: Recognize that a significant portion of individual variability arises from unique life experiences, personal choices, and metacognitive development, not solely genetics or family background.
• Immediate Action: Focus on Individual Journeys: When considering talent or achievement, look beyond innate ability and consider the specific, often idiosyncratic, paths individuals have taken, including deliberate practice and unique learning strategies.
• Longer-Term Investment: Cultivate Metacognitive Skills: Invest in understanding and developing personal approaches to learning, problem-solving, and emotional regulation, as these are key drivers of success, particularly in complex fields.
• Longer-Term Investment: Support Education and Deliberate Practice: Recognize that while education may not always "scale" in simple ways, the environment and opportunities for focused, persistent effort are crucial for developing specialized skills.
• Immediate Action/Longer-Term Investment: Seek Nuance over Certainty: Understand that human development is a complex, multi-factorial process. Avoid simplistic conclusions that attribute outcomes solely to genetics or environment. The "discomfort" of acknowledging this complexity now leads to a more accurate and empowering understanding of human potential later.
• Immediate Action: Question the "Perfect Experiment": Be skeptical of studies claiming to offer definitive, simple answers to complex human questions, especially when they rely on rare or imperfect "natural experiments."