Education Drives Twins-Apart IQ Differences
False Assumption: IQ differences of identical twins reared apart are significantly influenced by educational differences.
Written by FARAgent on February 10, 2026
In July 2025, researchers published an analysis in Acta Psychologica of IQs and schooling for 87 pairs of identical twins raised apart. They claimed educational differences significantly drove IQ gaps. The study used summaries of twin data to argue nurture effects.
Steve Sailer shared screenshots of the data in October 2025. He observed that 26 of 87 pairs, or 30 percent, attended the same school for part of their education. He pointed to pairs 71 and 72, Colombian twins switched at birth and raised in city or rural settings with 16 versus 5 years of schooling; city twins outscored rural ones by 22 and 7 points, averaging 14.5.
Critics note confounds like shared schooling undermine clean separation. The Colombian cases, though elegant accidents, represent tiny samples. Mounting questions arise about how strongly education explains differences given imperfect separation.
People Involved
- In the realm of twin studies, Jared C.
- Horvath and Katie Fabricant stepped forward with a claim. They published an article asserting that IQ differences among identical twins reared apart stemmed largely from variations in their education. These researchers pushed a nurture-based explanation in good faith. [1]
- Meanwhile, Nancy Segal, a specialist in twins, detailed cases from Colombia. She described pairs switched at birth, one raised in a rural setting and the other in an urban one, unaware of their twin status. Her work supplied data that seemed to bolster ideas about environmental impacts on IQ. [1]
- On the critical side, Steve Sailer entered the fray as an analyst. He shared screenshots of the data and pointed out issues like shared schooling among some pairs. Sailer played the role of a skeptic, warning about flaws in assuming true separation. [1]
▶ Supporting Quotes (3)
“IQ differences of identical twins reared apart are significantly influenced by educational differences
Jared C. Horvath and Katie Fabricant”— Data for the 87 Twins-Raised-Apart Pairs
“They are the two sets of Colombian twins described by Nancy Segal who got switched in the maternity award and each pair grew up thinking they were fraternal twins.”— Data for the 87 Twins-Raised-Apart Pairs
“I’ve responded to this article at some length in my previous Substack post”— Data for the 87 Twins-Raised-Apart Pairs
Organizations Involved
The journal Acta Psychologica played a key role in this story. It published summaries of IQ scores and schooling details for 87 pairs of twins raised apart. Through its peer-reviewed pages, the journal advanced the notion that nurture, via education, drove these IQ gaps. This act of dissemination lent academic weight to the assumption, even as questions lingered in quieter corners. [1]
▶ Supporting Quotes (1)
“Below are the screenshots of the summaries of IQs and schooling for the 87 pairs of identical twins raised apart that appear in
Acta Psychologica
, July 2025”— Data for the 87 Twins-Raised-Apart Pairs
The Foundation
The assumption took root in a dataset of 87 twin pairs. This collection appeared solid, with records showing correlations between schooling years and IQ differences. It fostered a belief in powerful nurture effects on intelligence. Yet critics argue it overlooked a detail: thirty percent of the pairs had attended the same schools, muddying the waters of separation. [1] Such oversights made the foundation seem credible at first glance, but growing questions surround its completeness.
▶ Supporting Quotes (1)
“Of the 87 pairs, 26 (30%) attended the same school for at least part of their educational careers.”— Data for the 87 Twins-Raised-Apart Pairs
How It Spread
The idea gained traction through academic channels. Publication in Acta Psychologica marked a turning point. The journal spread summaries of the 87 pairs' IQ and education data far and wide. This dissemination occurred in the usual way, via scholarly networks and citations, embedding the assumption in discussions of nature versus nurture. [1]
▶ Supporting Quotes (1)
“screenshots of the summaries of IQs and schooling for the 87 pairs of identical twins raised apart that appear in
Acta Psychologica
, July 2025”— Data for the 87 Twins-Raised-Apart Pairs
Downfall
Mounting evidence began to challenge the assumption's core. Analysis showed that 26 of the 87 pairs had gone to the same school. This revelation exposed incomplete separation, prompting critics to argue that education's influence on IQ differences might not be as isolated as claimed. [1] In Colombia, pairs numbered 71 and 72 offered a vivid example. Switched at birth, one twin in each pair grew up in a city with 16 years of schooling, while the other lived rurally with just five years. The urban twins averaged 14.5 IQ points higher. [1] Though elegant in design, this small sample highlighted limits, and growing questions surround whether such cases truly isolate education's role. The debate remains contested, with experts divided on the implications.
▶ Supporting Quotes (2)
“Many of these pairs were not as apart as you might assume. Of the 87 pairs, 26 (30%) attended the same school for at least part of their educational careers.”— Data for the 87 Twins-Raised-Apart Pairs
“It looks like one City Mouse outscored his Country Mouse twin by 22 points and the other by 7 for an average difference of 14.5 points.
Granted, that’s a tiny sample size, but it’s an incredibly elegant experimental design (except for not being designed at all and happening only through hospital incompetence).”— Data for the 87 Twins-Raised-Apart Pairs
Sources
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[1]
Data for the 87 Twins-Raised-Apart Pairsreputable_journalism