Schooling impacts IQ: The case of identical twins raised apart

A new twins study is making some waves.

Is IQ largely the result of nature or nurture? This question has a very long research tradition. Francis Galton developed the “classical twin study” to attempt to answer this question. We know that identical twins share virtually 100% of the same genetic material whereas fraternal twins share about 50% of the same genetic material. Given this information, twin studies imply that any traits more similar amongst identical twins rather than fraternal twins must have a stronger genetic basis.

However, in most of these studies, most twin pairs are raised by the same parents within the same home, which makes it difficult to separate out genetic and environmental influences on traits such as IQ. To solve this issue researchers have studied twins “reared apart”— separated at a young age and raised in a different household. The logic of these studies is that it’s easier to attribute similarities between identical twins to genetics, not the shared environment. These studies are rarer, however, since twins reared apart is a rare phenomenon.

Based on whatever studies have been done on this rare phenomenon, researchers have found a surprisingly strong correlation in the IQ scores of identical twins who were separated when they were young. A weighted average has been calculated at 0.75, with about an absolute average difference of 8.0 IQ points. In comparison, identical twins reared together typically differ by 6 IQ points, non-twin siblings differ by 12 points, and complete strangers differ by 16.9 points.

These findings have led researchers to conclude that IQ is largely determined by genetics. However, there is a big issue with these studies: differences in schooling. The majority of twin studies are averaged from large groups of identical twin pairs reared apart and therefore cannot account for differences in schooling.

Enter a new analysis which is making some waves. Using biographical information, psychologists Jared Horvath and Katie Fabricant attempted to gather data from every available twins reared apart study published in the academic literature one the last century that included schooling information. The classified whether twins reared apart had similar educational experiences or dissimilar educational experiences. In total, they analyzed data on 87 twin pairs reared apart. What did they find?

First, they found an absolute IQ differential of 8.6 points among identical twins reared apart, which is consistent with what others have reported. So far so good. Next they split groups into educationally similar and dissimilar groups. Their analysis revealed that educationally similar identical twin pairs showed an absolute IQ differential of 5.8 points, making them indistinguishable from identical twins raised together. In contrast, the correlation for the educationally dissimilar pairs showed an absolute IQ differential of 12.8 points.

In other words, IQ differences grew in tandem with schooling differences. The fact that identical twins reared apart with near identical schooling showed an IQ pattern similar to identical twins reared together suggests that the aspects of schooling which impact IQ must be general enough to generalize beyond specific teachers or peer groups. Also, the fact that identical twins reared apart with different schooling showed an IQ pattern similar to strangers suggests that aspects of school which impact IQ must in part due to curricula, pedagogy, and the duration of schooling.

These findings are important and challenge some prior interpretation of twin data. Most twin researchers have used the prior findings as evidence that IQ is largely genetically determined, but since the correlations in IQ among twins can drop so dramatically based on schooling suggests that simple models of genetic influence may need to be taken into account.

A major limitation is that they were only able to find 10 twins reared apart in the “very dissimilar education” group. That is the nature of these sort of studies, as it’s so hard to find such twin pairs.

Also, while schooling seems to play a role, schooling is not the only environmental influence on IQ. Nutrition, a decrease in the prevalence of infectious diseases, and an increase in visual media, parental education, the number of books in the home, and SES all have shown a correlation with IQ.

It should be noted that while these findings suggest that education differences matter for IQ, it has been surprisingly difficult for any large-scale interventions to systematically and reliably change the IQ of children. Educators still don’t understand exactly what needs to be done to change IQ, even though we can say that education differences matter. It’s still a great mystery what particular features of schooling matter the most for influencing IQ scores.

Finally, I’d like to point out that these findings don’t suggest that IQ is entirely environmentally determined. The bulk of the evidence on studies looking at the development of IQ among the general population (not just among twins) suggests that the development of IQ is the result of nature and nurture interacting in complex ways.

What is important about this new analysis is that it calls into question the strong genetic interpretation of past twins reared apart studies and suggests that researchers may have ignored the important role schooling can play in the development of IQ.