Are Some Destined to Be Smart?
Twin studies show how genes matter, but intelligence is still ours to shape.
Posted October 2, 2025 | Reviewed by Devon Frye
One of the most enduring mysteries of the mind is intelligence , particularly where it comes from and whether we can increase it. Philosophers have wrestled with the question for millennia, and the past century has brought a flurry of scientific attempts to pin it down. Yet firm answers remain elusive.
We know intelligence exists, and humans more than any other species revel in the cognitive marvels of the brain. Although individual scores tend to stay relatively stable across one’s life, we also know IQ is by no means completely fixed. For example, studies like Deary et al.'s (2000) show how correlations between childhood and later-life scores are often in the 0.60-0.70 range, leaving much space for development over age. And yet, the same studies show how the impact of genes on intelligence is an inescapable fact. But does that mean that some people are destined to be smart?
The Nature vs. Nurture Debate That Never Should've Happened
Let’s begin with something less fraught than intelligence: height.
No one doubts that genes affect height. Twin and family studies often show very strong correlations in the 0.90 range, with identical twins tending to be nearly identical in stature.
In short, genetics clearly sets the stage for how tall we can grow in any given environmental context. All else being equal, DNA would run the show entirely, but no double helix ever gets the exact same treatment, not even in the same body, where each cell presents its own environment.
This brings us to one of the most useful analogies for understanding the interplay between nature and nurture. Genes set the stage, while the environment decides how (and whether) the play unfolds. Applied to intelligence, genes can provide the building blocks for fast synapses, exceptional plasticity, and other components of cognitive performance—but they do not ensure that those components are deployed optimally.
What Twin Studies Have Taught Us About Intelligence
Twin studies, especially those championed by behavioral geneticists like Robert Plomin, provide perhaps the clearest window into understanding how the stage gets set.
The closest thing we have to a gold standard is comparing monozygotic (identical) twins with dizygotic (fraternal) twins, who share only about half their segregating genes. If we find that identical twins resemble each other far more in IQ than fraternal twins do, the finding points strongly to genetic influence.
And this is exactly what twin studies have shown. When researchers track twins across development, the genetic signal in intelligence grows stronger with age. In one large meta-analysis of more than 11,000 pairs, Haworth and colleagues (2009) reported heritability estimates for identical twins' cognitive ability in young adulthood to be at 0.82, almost double that of fraternal twins.
Equally fascinating is how the study found that correlations developed over time, from 0.41 in childhood (average age nine), 0.55 in adolescence (age twelve), and 0.66 by late teenage years (age seventeen). In other words, as children grow, their cognitive differences increasingly mirror their genetic differences.
Perhaps the most striking evidence comes from the rare cases of identical twins reared apart, life’s accidental natural experiment. Despite growing up in different families, in different towns and contexts, their IQs often correlate in the 0.70–0.80 range, nearly as high as twins raised together. The famed Minnesota Study of Twins Reared Apart, led by Bouchard and colleagues (1990), produced many of these examples, showing how powerfully the genetic blueprint holds even when environments diverge.
The similarity between identical twins reared apart is striking, given how correlations of .70 or higher are rare in the behavioural sciences, where most effects barely clear .20 or .30. It’s no wonder these findings have been taken as proof that intelligence is overwhelmingly genetic.
But that view skips the other side of the ledger entirely in its rush to write a catchy headline.
Even with the same DNA blueprint, as much as 25–30 percent of the variation remains environmental. In other words, a full third of the picture is still shaped by context, chance, and experience.
Genes matter enormously, but they do not write the whole story.
So, Are Some Destined to Be Smart?
Twin studies show how genes clearly shoulder much of the load when it comes to cognitive potential. They shape the scaffolding on which intelligence is built, but intelligence is by no means a fixed destiny.
The environment, chance, and our own choices still write a large part of the script. Up to one third of it, in fact. DNA may hand us the blueprint, yet how that design is realised depends on the classrooms we sit in, the mentors we meet, the challenges we take on, and the habits we cultivate. Even in identical twins, nature’s closest approximation of a controlled experiment, more than a fifth of the variation in intelligence comes from forces beyond their shared DNA.
The lesson points less to inevitability and more to possibility, if only we choose to see it that way. Our biology sets the stage, but the play is still ours to direct.
Deary, I. J., Whalley, L. J., Lemmon, H., Crawford, J. R., & Starr, J. M. (2000). The stability of individual differences in mental ability from childhood to old age: Follow-up of the 1932 Scottish Mental Survey. Intelligence, 28 (1), 49–55.
Macgregor S, Cornes BK, Martin NG, Visscher PM. Bias, precision and heritability of self-reported and clinically measured height in Australian twins. Hum Genet. 2006 Nov;120(4):571-80. doi: 10.1007/s00439-006-0240-z. Epub 2006 Aug 25.
Haworth CM, Wright MJ, Luciano M, Martin NG, de Geus EJ, van Beijsterveldt CE, Bartels M, Posthuma D, Boomsma DI, Davis OS, Kovas Y, Corley RP, Defries JC, Hewitt JK, Olson RK, Rhea SA, Wadsworth SJ, Iacono WG, McGue M, Thompson LA, Hart SA, Petrill SA, Lubinski D, Plomin R. The heritability of general cognitive ability increases linearly from childhood to young adulthood. Mol Psychiatry. 2010 Nov;15(11):1112-20. doi: 10.1038/mp.2009.55. Epub 2009 Jun 2. PMID: 19488046; PMCID: PMC2889158.
Bouchard, T. J., Lykken, D. T., McGue, M., Segal, N. L., & Tellegen, A. (1990). Sources of human psychological differences: The Minnesota Study of Twins Reared Apart. Science, 250(4978), 223–228.
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T. Alexander Puutio, Ph.D., teaches at Harvard and is an organizational performance expert exploring how people and organizations flourish through curiosity, range, and purposeful leadership.
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