Environment Or Genetics?
Maybe Environment Is The Driver
Ever since I watched The Three Stooges short Half-Wits Holiday as a child, I have been aware of the debate between environment and genetics in explaining human achievement. Over the past month, I have encountered two research findings that are reshaping my perspective on Intelligence Quotient (IQ). IQ has long been a subject of fascination and debate, serving as a proxy for cognitive ability and a predictor of outcomes in education, occupation, and health. Historically, IQ research has oscillated between emphasizing genetic determinism and environmental influence, with twin studies and population comparisons providing critical insights. Recent research has further nuanced this discourse, revealing that IQ is neither solely genetic nor entirely environmental but emerges from a dynamic interplay of both.
Last month at the AEI Old Parkland Conference, I heard Richard H. Thaler, the 2017 Nobel Prize in Economics laureate, discuss IQ studies involving twins. His key takeaway was that IQ is not fixed. Environment has a significant impact on IQ. More recently, while listening to the Conversations with Tyler podcast, I learned from neuroscientist Theodore Schwartz that IQ has limited value in so-called high-intellectual jobs.
The stereotype that neurosurgeons and rocket scientists possess superior intelligence is deeply ingrained, often encapsulated in the phrase "It's not rocket science" or "It doesn't take a brain surgeon." However, a study published in the British Medical Journal (BMJ) in December 2021 challenges this assumption, providing evidence that the cognitive abilities of these professionals are not significantly different from those of the general population. Conducted by Chari et al., this study involved 329 aerospace engineers, 72 neurosurgeons, and over 18,000 members of the British public, who completed the Great British Intelligence Test (GBIT) on the Cognitron platform. The test assessed multiple cognitive domains, including planning, reasoning, working memory, attention, and emotion processing.
The findings were striking: neither aerospace engineers nor neurosurgeons demonstrated overall cognitive superiority compared to the general population. Neurosurgeons showed a slight advantage in problem-solving speed, potentially attributable to the fast-paced demands of their profession, but they exhibited slower memory recall. Aerospace engineers scored higher in attention and mental manipulation tasks, such as visualizing object rotations, while neurosurgeons excelled in semantic problem-solving, like defining rare words. However, these differences were minor and did not indicate a broad intellectual edge. The researchers concluded that cognitive diversity exists within and across occupational groups, and professional expertise reflects specific cognitive strengths rather than universal intelligence.
This study has significant implications. It debunks the myth that certain professions inherently require exceptional IQs, highlighting that task-specific skills and training may enhance performance in particular domains without elevating overall IQ. For example, the rapid decision-making required in neurosurgery may hone problem-solving speed, but this does not translate to superior general intelligence. Similarly, the mental manipulation skills of aerospace engineers reflect the demands of their work rather than an innate cognitive advantage. These results align with broader trends in IQ research, which increasingly emphasize the role of environmental factors—such as education, training, and professional experience—in shaping cognitive performance. While genetics may provide a foundation for cognitive potential, the expression of that potential is heavily modulated by environmental opportunities and demands.
Twin studies have been a cornerstone of behavioral genetics, leveraging the genetic similarity of monozygotic (identical) twins, who share nearly 100% of their DNA, and dizygotic (fraternal) twins, who share about 50%, to disentangle genetic and environmental influences. Recent longitudinal twin studies, particularly those involving twins reared apart, have provided compelling evidence that environment plays a massive role in IQ, challenging earlier assumptions of high genetic determinism.
A 2024 study by Segal et al., published in Personality and Individual Differences, examined three groups: young Chinese monozygotic twins reared apart due to China's former One-Child Policy, adult Danish monozygotic twins reared apart, and "virtual twins" (same-age, unrelated siblings reared together). The study measured IQ using age-appropriate Wechsler tests at two time points, allowing researchers to track changes over time. The Chinese twins, tested at mean ages of 10.69 and 13.93 years, showed increasing IQ resemblance over time (intraclass correlations rose from 0.51 to 0.81), suggesting that genetic factors become more influential as individuals age. Similarly, the Danish twins, tested at a mean age of 51.42 years, showed high IQ resemblance (correlations of 0.64 and 0.74). In contrast, virtual twins exhibited decreasing IQ resemblance, indicating that shared environments have a limited long-term impact on IQ.
These findings support the idea that the heritability of IQ increases with age, from about 20% in infancy to potentially 80% in adulthood. This increase is attributed to the amplification of genetic predispositions as individuals select environments that align with their innate abilities (a process known as gene-environment correlation). However, the study also highlights the significant role of environment in early development. For the young Chinese twins, environmental differences due to being raised in separate households initially led to greater IQ variability, which diminished as genetic factors became more pronounced with age. This suggests that early environmental interventions can have a substantial impact on IQ, even if genetic influences dominate later in life.
Another pivotal study, published in 2022 in Personality and Individual Differences, examined a pair of monozygotic twins raised apart in South Korea and the United States. This case study revealed significant differences in cognitive abilities, with the twin raised in South Korea scoring 16 points higher in IQ, particularly in perceptual reasoning and processing speed. The researchers attributed this discrepancy partly to the U.S.-raised twin's history of concussions, which are known to impair cognitive function. Despite these differences, the twins showed remarkable similarities in personality traits (e.g., high conscientiousness, low neuroticism) and self-esteem, underscoring the strong genetic basis of these traits.
This study demonstrates how environmental factors—such as physical trauma or cultural context—can significantly alter IQ outcomes, even in individuals with identical genetic makeup. The 16-point IQ gap is notably larger than the typical 7-point difference observed among identical twins raised together, suggesting that extreme environmental disparities (e.g., different countries, healthcare systems, or life experiences) can override genetic predispositions to a significant extent. The fact that the twins' mental health and self-esteem remained similar despite these environmental differences further complicates the narrative, indicating that some traits are more resilient to environmental variation than others.
Recent twin studies have also explored gene-environment interactions, particularly in the context of socioeconomic status (SES). A 2007 study by Harden, Turkheimer, and Loehlin found that among adolescents, the heritability of cognitive aptitude varied by family income. In higher-income families, genetic influences accounted for about 55% of variance in cognitive aptitude, with shared environmental influences contributing 35%. In lower-income families, these proportions reversed, with shared environment accounting for 45% and genetics 39%. This suggests that in impoverished environments, environmental factors—such as nutrition, education, and stress—can suppress genetic advantages, leading to lower IQ expression. Conversely, in enriched environments, genetic potential is more fully realized, resulting in higher heritability estimates.
A 2003 study by Turkheimer et al. reported that the heritability of IQ is nearly zero in impoverished households, where environmental risks dominate cognitive outcomes. These studies highlight the concept of "range of reaction," where genetic predispositions interact with environmental conditions to produce a range of possible outcomes. For example, a child with a genetic predisposition for high IQ may achieve that potential in a supportive environment but fall short in a deprived one. This underscores the critical role of early interventions, such as access to quality education and healthcare, in maximizing cognitive potential.
Emerging research on epigenetics provides a molecular perspective on how environment influences IQ at the genetic level. Epigenetics refers to modifications in gene expression that do not alter the DNA sequence but can be influenced by environmental factors like diet, stress, or trauma. A 2018 article in The Atlantic discusses how epigenetic changes can account for differences in identical twins, even those with identical DNA. For instance, a 2016 study of twins discordant for rheumatoid arthritis found that smoking-induced epigenetic changes altered DNA function in regions linked to inflammation, contributing to disease onset in one twin but not the other.
While direct evidence linking epigenetics to IQ is still emerging, these studies suggest that environmental factors can modulate genes related to cognitive function, potentially explaining IQ variability in genetically identical individuals. This is particularly relevant for twins reared apart, where differences in upbringing—such as exposure to pollutants, educational opportunities, or emotional stress—may induce epigenetic changes that affect cognitive outcomes. The field of epigenetics thus bridges the gap between genetics and environment, offering a mechanism through which nurture shapes nature at the molecular level.
The convergence of findings from the BMJ study and recent twin research paints a complex picture of IQ. The lack of significant IQ differences between neurosurgeons, rocket scientists, and the general population suggests that high-level professional achievement does not necessarily require exceptional cognitive ability but rather a combination of specific skills, training, and environmental opportunities. This challenges the notion that IQ is a fixed, genetically determined trait that dictates professional success. Instead, it highlights the role of environmental factors in shaping cognitive performance.
These findings have important implications for policy and practice. The significant environmental influence on IQ, especially in early childhood, underscores the need for interventions that address disparities in education, nutrition, and healthcare. Programs targeting low-SES communities could mitigate the environmental barriers that suppress cognitive potential, allowing genetic predispositions to flourish. Similarly, the recognition that professional success in fields like neurosurgery or aerospace engineering does not require exceptional IQ can democratize access to these careers. These findings challenge the notion of IQ as a fixed, genetically determined trait, highlighting instead its malleability and context-dependence.
References
Chari, A., et al. (2021). Cognitive abilities in neurosurgeons and aerospace engineers compared with the general population. BMJ, 375.
Segal, N. L., et al. (2024). Groundbreaking study reveals the impact of genetics on IQ scores over time. Personality and Individual Differences.
Segal, N. L., et al. (2022). A new study of monozygotic twins raised apart in South Korea and the United States. Personality and Individual Differences.
Harden, K. P., Turkheimer, E., & Loehlin, J. C. (2007). Genotype by environment interaction in adolescents’ cognitive aptitude. Behavior Genetics.
Turkheimer, E., et al. (2003). Socioeconomic status modifies heritability of IQ in young children. Psychological Science.
The Atlantic. (2018). Identical twins hint at how environments change gene expression.
Plomin, R., et al. (2014). Genetics and intelligence differences: five special findings. Molecular Psychiatry.