Wednesday, October 22, 2025

New Twin Study Clashes With "Brains Make Minds" Claim

 A recent press release has a title of "Major IQ differences in identical twins linked to schooling, challenging decades of research." We read of a scientific paper with results shocking to those who assumed that intelligence is a product of the brain. 

Two scientists compiled a dataset involving 87 twin pairs. The dataset included IQ scores and data on the educational experience of the twins. 52 pairs of twins with similar schooling had an average IQ difference of about 6 points. 25 pairs of twins with "somewhat similar" schooling had an average IQ difference of about 12 points. For the ten pairs of twins with "very dissimilar" schooling, the average IQ difference was 15 points. This is similar to the average IQ difference between two randomly selected people, which is about 17 points. 

Fifteen points is a big difference on an IQ score. A graph showing IQ scores as found in a human population will look like the graph below, with the most common numbers being found in the middle. 


We may presume that twins have identical brains. So we see a big average difference (about 15 points) between the intelligence levels of twins with identical brains, when such twins have very different educations. This is a result that clashes with claims that the brain is the source of someone's mind.  It would seem that if such claims are true, twins with identical brains should tend to have IQ scores almost identical. 

A recent article in The Atlantic by psychology professor Erik Turkheimer is entitled "Your Genes Are Simply Not Enough to Explain How Smart You Are." We read this:

"We do not understand the genetic or brain mechanisms that cause some people to be more intelligent than others. The more we have learned about the specifics of DNA associated with intelligence, the further away that goal has receded....If anything, we are further away now than in 2018 in knowing 'basically what's going on' with genetic influences on intelligence....All of the most direct methods of searching for IQ genes were unsuccessful."

Whenever people make statements like this, we should tend to regard them as confessions which undermine or cast doubt on one of the underlying assumptions of the confessor. When someone confesses that he does not know how we will be able to do something, we should regard that as a strong reason for doubting that he will be able to do that thing. When a prosecutor confesses that he does not know how a murder defendant committed a claimed murder, we should doubt that such a person committed such a murder. And when some expert confesses that we do not know how Factor X causes Result Y, we should regard this as being a reason for doubting that Factor X causes Result Y.  So when we hear people confess things such as "we do not understand the genetic or brain mechanisms that cause some people to be more intelligent than others," we should then doubt very much that there are "genetic or brain mechanisms that cause some people to be more intelligent than others." 

puzzled neuroscientist

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