For more than a century, scientists have debated whether intelligence is mostly a product of nature or nurture. Identical twins—those born with the same genetic blueprint—have long been the perfect test case.
When twins are raised apart, researchers can compare their differences to see how much of who we are comes from our DNA and how much comes from the world around us. Now, a sweeping new analysis of all known identical twins raised apart suggests that education plays a much bigger role in shaping intelligence than previously believed.
Cognitive neuroscientist Jared C. Horvath of the English Schools Foundation Center for Research and developmental scientist Katie Fabricant of the University of Wisconsin reviewed every available case of identical twins reared apart that included both IQ scores and educational details. Their analysis, published in Acta Psychologica, pooled data from 87 twin pairs collected over the last hundred years.
Past research often combined data from many twin studies and reported an average IQ gap of around eight points, with a high “intraclass correlation coefficient”—a measure showing strong similarity in IQ. Those findings supported the idea that intelligence is mostly inherited. But Horvath and Fabricant noticed something missing from the equation: the details of each twin’s education.
“When you lump all twins together, you lose sight of the individual stories,” Horvath explained. “Schooling isn’t just a background variable—it’s one of the strongest environmental influences on cognitive development.”
To dig deeper, the researchers grouped the twins by how similar or different their educational paths were. They looked at three main dimensions:
Each pair received an “education difference” score, placing them into one of three categories: very similar, somewhat dissimilar, or very dissimilar schooling. Then the team calculated both the average IQ difference and the IQ correlation for each group.
The patterns that emerged were striking. Among twins who had very similar schooling, the IQ gap averaged just 5.8 points, with a correlation of 0.87—almost identical to twins raised together in the same home. In other words, when education was nearly identical, so were their IQs.
For twins with somewhat different schooling, the IQ gap more than doubled to 12.1 points, and the correlation fell slightly to 0.80. And for those who experienced drastically different schooling—different systems, locations, and lengths of education—the IQ difference widened to 15.1 points on average, while the correlation dropped sharply to 0.56.
That’s nearly the same IQ difference seen between two unrelated people. In short, the further apart their schooling, the further apart their measured intelligence became.
For decades, twin studies have been used to argue that genes dominate the story of intelligence. A consistently high correlation between identical twins’ IQs—around 0.8—was seen as strong proof of heredity’s grip. This new analysis challenges that assumption.
Rather than a fixed number, Horvath and Fabricant’s work suggests the twin IQ correlation fluctuates with education quality and similarity. In families or societies with greater educational inequality, those numbers may vary even more.
The takeaway? Genes set the potential, but education determines how much of that potential is realized. Identical DNA doesn’t guarantee identical outcomes when life experiences—especially schooling—diverge.
Twin separations are rare and often heartbreaking, which is why there are so few cases to study. Only 10 twin pairs in the dataset fell into the “very dissimilar” schooling group. That small sample size limits how far the results can be generalized, but the trend remains compelling.
Historically, researchers focused on averaging results from large twin datasets, which blurred the details of each twin’s upbringing. This study takes the opposite approach—zooming in on individual cases. By doing so, it highlights how much life experience can alter even the most genetically similar minds.
Fabricant said she hopes the work encourages more sharing of individualized twin data to improve future research. “If we want socially meaningful insights about intelligence,” she noted, “we have to move beyond averages and look at the real-world differences that shape people’s lives.”
IQ, originally developed in 1905 to identify students needing extra help, has often been treated as a stable trait. But global trends tell another story. Over generations, IQ scores have steadily risen across countries—a phenomenon many experts attribute to improved education and access to information.
This new study adds another layer to that understanding, suggesting that even among those with identical genes, education can widen or narrow the cognitive gap. That finding supports a growing body of evidence linking the length and quality of schooling to gains in IQ.
“The fact that identical twins can end up with IQs as different as strangers if their schooling diverges tells us something powerful,” said Horvath. “Education doesn’t just add knowledge—it reshapes potential.”
Of course, education isn’t the only factor influencing IQ. Home environment, health, motivation, and socioeconomic conditions all play roles. The researchers are careful not to claim that schooling alone determines intelligence. Yet the data suggest education exerts an unusually strong pull—strong enough to rival or even overshadow genetics under certain conditions.
Horvath and Fabricant view their findings as a step toward more nuanced science, one that blends biological and environmental perspectives. “Hopefully,” they wrote, “this paper can serve as one brick in the larger wall of understanding being built.”
The study underscores how critical education is—not only as a social equalizer but also as a driver of human potential. If differences in schooling can make genetically identical people diverge by up to 15 IQ points, then expanding access to high-quality education could help reduce cognitive disparities across society.
For policymakers, the message is clear: education reform isn’t just about fairness; it’s about unlocking potential. For teachers and parents, it reinforces how deeply learning environments shape intellectual development.
And for researchers, it’s a call to rethink old assumptions about intelligence, genes, and what truly makes minds alike or different.
Research findings are available online in the journal Acta Psychologica.