At first glance, two identical twins share more than just their looks. Same genetic heritage, similar intrauterine development, sometimes the same facial expressions or intonations. Yet when they grow apart, some unexpected differences appear. One excels in logic, the other struggles to follow. One develops a spirit of synthesis, the other a fragmented memory. Behind these differences, a question arises insistently. What does the variation in IQ really reveal among supposedly identical individuals?
Identical minds, divergent intellectual destinies
Monozygotic twins have always fascinated researchers because they offer a unique opportunity to distinguish what is biological from what is environmental. For more than a century, these studies have consolidated the idea that intelligence is strongly inherited. Reference work, such as that of Thomas Bouchard in the 1990s, has shown that twins raised separately maintain surprisingly close IQs, suggesting a major genetic influence.
But recent work by Jared Horvath and Katie Fabricant, published in 2025 in Acta Psychologica, reevaluates these conclusions. By pooling all available individual data on 87 pairs of identical twins raised apart, researchers observed that education profoundly changes this equation. When twins followed similar educational paths, their average IQ difference did not exceed six points. Conversely, those who experienced very distinct educational contexts show a gap of up to fifteen points. In other words, two genetically identical individuals can develop intellectual performances as different as those of strangers.
What IQ variation reveals about the impact of the education system
This variation in IQ cannot be reduced to the quality of a teacher or the richness of a program. It reflects a cumulative effect of school experience on cognitive abilities. According to Horvath and Fabricant, each additional year of schooling would result in an average gain of four IQ points, an estimate consistent with the findings of a large meta-analysis conducted by Stuart Ritchie and Elliot Tucker-Drob in 2018.
The history of psychology supports this observation. Since the development of the first intelligence test in 1905 by Alfred Binet, average scores have continued to increase over the century. This phenomenon, known as the “Flynn effect,” has been attributed to multiple factors: better nutrition, healthier health environments, increased exposure to visual media. However, the strongest correlation remains that between average education level and changes in global cognitive performance. According to data compiled by Horvath and Fabricant, the IQ progression curve almost perfectly follows that of the number of years of school on a global scale.
Thus, teaching acts well beyond the transmission of knowledge. It shapes memory, logic and mental flexibility, dimensions long perceived as innate. Even genes don't seem to act in isolation. Work in epigenetics suggests that lived experiences (such as intensive learning or intellectual stimulation) can influence the expression of certain genes involved in cognitive functions.
What if intelligence was constructed more than it was inherited?
These results resonate as an invitation to rethink the hierarchy between nature and culture. The researchers point out that twins who received a similar education have cognitive profiles as similar as those of twins raised together, while those who attended different schools gradually move apart, until they resemble simple strangers. This shift thus highlights the social and cumulative dimension of intelligence.
The school does not just evaluate potential, it sculpts it. Through exposure to intellectual challenges, through the diversity of cultural contexts and pedagogies, it models the brain as an organ of adaptation. This is perhaps the most striking lesson of this study. Intelligence is not a fixed inheritance, but a trajectory shaped by experiences. As Horvath and Fabricant summarize in ScienceAlert, each educational journey leaves a unique imprint on mental abilities, so deep that it can erase the similarities written in DNA.
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