A new analysis of studies spanning the last century finds that differences in schooling appear to explain a large portion of the IQ gap between identical twins who were raised in separate homes. The research suggests that when these twins receive similar educations, their IQ scores are nearly as alike as identical twins raised together, but when their schooling is very different, their IQs can be as dissimilar as those of unrelated strangers. The findings were published in the scientific journal Acta Psychologica.
The study was conducted by researchers Jared C. Horvath of the English Schools Foundation Center for Research and Katie Fabricant of the University of Wisconsin. They undertook this research to re-examine one of the most classic methods used to explore the “nature versus nurture” debate: the study of monozygotic, or identical, twins reared apart. Since these twins share nearly identical genetic material but grow up in different environments, they offer a unique window into the influences of genes and life experiences on human traits like intelligence.
For decades, research on this topic has generally concluded that genetics play a very strong role in determining intelligence as measured by IQ tests. Large studies have typically combined the data from many twin pairs and reported an average IQ difference of about 8 points and a high degree of similarity, measured by a statistic called an intraclass correlation. This high correlation has often been used to argue that intelligence is a largely stable, inborn trait.
The authors of the new paper identified a potential problem with this long-standing conclusion. The historical practice of averaging data from dozens of twin pairs, known as amalgamation, masks the specific life experiences of each individual. One of the most powerful environmental influences known to affect IQ scores is education. Extensive research has shown a causal link between the amount of schooling a person receives and their performance on IQ tests. By lumping all twin pairs together, past studies may have obscured the true impact of educational differences on their cognitive abilities.
To investigate this possibility, the researchers embarked on an exhaustive search of the academic literature published over the last century. They looked for every available study that contained individualized data on monozygotic twins who had been reared apart. To be included in their new analysis, a case had to provide not only the IQ score for each twin but also biographical information about their schooling.
After screening thousands of articles, they compiled a dataset of 87 twin pairs from 19 different studies and two personal communications with other researchers. The authors believe this collection represents the entirety of non-amalgamated data available in the field.
With this unique dataset, Horvath and Fabricant developed a system to quantify the educational differences between each pair of twins. They created an “Ed Diff” score based on three key dimensions of schooling known to impact academic achievement. These were differences in curriculum, such as attending a public versus a Catholic school; differences in pedagogy, such as being educated in different countries or states with distinct teaching frameworks; and differences in duration, meaning the total number of years each twin spent in school.
Based on these scores, the researchers divided the 87 pairs into three categories. The first group had “similar” educational experiences. The second had “somewhat dissimilar” schooling, and the third had “very dissimilar” educational backgrounds. They then analyzed the IQ differences and the intraclass correlation for each group, using the same measures that have been standard in the field for decades. The intraclass correlation is a statistic that measures how strongly individuals within the same group, in this case a twin pair, resemble each other.
First, the researchers analyzed the entire group of 87 pairs together, to see if their dataset was comparable to previous aggregated studies. They found an average absolute IQ difference of 8.6 points and an intraclass correlation of 0.80. These figures aligned closely with the historically reported averages, confirming their sample was representative of the field.
The results changed dramatically when they analyzed the groups based on educational similarity. For the 52 pairs with similar schooling, the average IQ difference was just 5.8 points, and the intraclass correlation was a very high 0.87. This finding makes them almost indistinguishable from monozygotic twins who are raised together in the same home, who typically differ by about 6.0 IQ points.
For the 25 pairs with “somewhat dissimilar” schooling, the average IQ difference grew to 12.1 points, with an intraclass correlation of 0.80. This level of difference is more comparable to that seen between non-twin siblings raised in the same family.
The most striking finding came from the 10 pairs with “very dissimilar” educational experiences. In this group, the average IQ difference was 15.1 points. This gap is approaching the average difference seen between two randomly selected, unrelated individuals, which is about 17 points. The intraclass correlation for this group dropped to 0.56, a value much closer to what is observed between non-twin siblings. The pattern was clear: as the differences in schooling between the identical twins increased, the differences in their IQ scores also increased substantially.
The authors note some limitations to their work. The group with “very dissimilar” education contained only 10 twin pairs. While this represents all such published individual data from the last century, it is a small sample size. It is possible that more twin pairs with significant educational differences exist, but their data has only been included in historical aggregate analyses and has not been made public. The researchers suggest that until this individualized data is shared, it will be impossible to know for certain the full extent of schooling’s impact.
This analysis brings into question the historic use of a single intraclass correlation figure to represent the genetic influence on IQ. The results show that this statistic is not a fixed number but can change significantly depending on environmental factors like education. The study’s authors propose that the field should shift away from large-scale aggregate studies, which are unlikely to yield new information.
Instead, they recommend a focus on in-depth case studies of individual twin pairs to better understand how specific life experiences shape psychological traits. They also issue a call for researchers who conducted past studies to share their de-identified, individualized data. This practice, they argue, is essential for ensuring that socially important theories about genetics and intelligence are as accurate and well-supported as possible.
The study, “IQ differences of identical twins reared apart are significantly influenced by educational differences,” was published in July 2025.
