Do Twins-Raised-Apart IQ Studies Prove Nature Over Nurture?

A new paper reveals IQ scores and years of schooling for each of 87 pairs of separated identical twins.

Identical twins raised apart (TRA) are the californium of human science nature vs. nurture studies. Only a few hundred pairs (and a small number of sets of triplets, like in the 2018 documentary Three Identical Strangers) have ever been studied by scientists.

Still, we have enough identical twins raised apart who have been carefully researched to discover that their IQs tend to be fairly similar, a major piece of evidence in that most contentious front of the Nature vs. Nurture debates.

On the other hand, I’ve long tried to think through the mechanics of how identical twins would wind up being raised apart. And it strikes me that split-up twins probably wouldn’t very often wind up in radically different Prince and Pauper-type environments. So, I’ve long been leery of the view that identical twin studies provide slam dunk final proof that Nature matters much more than Nurture when it comes to IQ.

There are some big differences in possible environments in this world, and I’m not sure that identical twins randomly get assigned to them when they get separated. For example, back in 2002, I pointed out that Lynn and Vanhanen found an average IQ in Sub-Saharan Africa of 70 compared to 85 for African-Americans and that that suggests that nurture matters for IQ when it’s as radically different as America vs. Africa. Has there ever been a twin study where some identical twins get raised in Bedford-Stuyvesant and the others in the African bush?

Note that for a majority of identical twins reared apart who have been studied in published academic articles, individual data such as IQ scores and years of schooling have not been released, perhaps for privacy reasons. Instead, just aggregated data has been published.

However, a fascinating new study recounts the individual IQ scores of 87 previously published pairs of identical twins raised apart along with some biographical data such as the amount and type of years of schooling for each:

IQ differences of identical twins reared apart are significantly influenced by educational differences

Jared C. Horvath, Katie Fabricant

Below I’ve posted their 87 pairs so you can get a sense of what identical twins raised apart are like.

I’ve looked through the brief listings for all 87 pairs and compared which twin got more years of schooling with which got a higher IQ score. In 13 cases, I can’t figure out how many years of schooling they got, and in 33 cases they got the same number of years (and averaged IQ differences of only 5.7 points). Among the 41 pairs of separated twins with different number of years of schooling, in 34 pairs the twin with more years of education scored higher (an average of 11.1 points higher among those pairs with a positive correlation of quantity of education and IQ) versus only 7 pairs where the twin with less education scored higher (an average difference of 10.1 points among those with a negative correlation).

For some reason, the authors don’t do this simple analysis, but they do some more complicated things that I will discuss in detail below the paywall over more than a couple of thousand words:

Paywall here.