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Mind

Genes for Intelligence - Back to Square One

Neuroskeptic iconNeuroskepticBy NeuroskepticDecember 13, 2011 7:10 PM

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Here's a paper - soon to appear in Psychological Science - which says that Most Reported Genetic Associations with General Intelligence Are Probably False Positives

The authors tried to replicate published associations between particular genetic variants (SNPs) and IQ (specifically the g factor). They looked at three datasets, a total of about 10,000 people, and didn't confirm any of the 12 associations.

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As Razib Khan says in his post on this, "My hunch is that these results will be unsatisfying to many people." I'd go further and say that no-one will be happy with these.

For those who believe that IQ is purely environmental and not genetic, any satisfaction they might feel will be short lived because these authors did replicate the recent finding that genetic variants explain about 50% of the variance in IQ. Looking at all SNPs together, there was a strong correlation between "genetic similarity" and similarity in IQ. That independently confirms what the much-criticized twin studies of IQ said - IQ is about 50% heritable.

But for people who do believe in the genetics of intelligence, this shows us that we have no idea what the genes are, and that everything published so far has been pretty much for naught.

There's another implication. We actually do know of many "IQ genes" in that we know genes that, when mutated, cause mental retardation (very low IQ).

Now many researchers have hoped that if a certain gene causes you to have an IQ of, say, 50 when it's completely deleted by a mutation, then more subtle variants in that gene would have minor effects on IQ. Maybe a variant that reduces expression of the gene by 10% would knock off 5 IQ points.

In other words, if big mutations cause big phenotypes, then small mutations in the same place ought to cause small phenotypes. It seems to make sense - but today's IQ literature shows that it's just not true.

That's not just a problem for IQ though. Take autism or ADHD, we know that there are rare, severe mutations that cause these conditions. Many people are hoping that common variation in the same genes might also be interesting - but if IQ is anything to go by, it won't be.

Perhaps this is not so surprising. Breaking your neck and becoming paraplegic is going to seriously impair your ability to play baseball. That doesn't mean that normal variation in baseballing skill has much to do with minor neck injuries.

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Chabris, C. F. et al (2011). Most Reported Genetic Associations with General Intelligence Are Probably False Positives Psychological Science

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