I’m almost done my post on Angela Merkel, but I thought I would do a quick post about genetics and IQ.  Commenter Afrosapiens asked how we know there are 10,000 genes (actually genetic variants) for intelligence, instead of say 13,000, 15,000 or 20,000.  I found an article by Dr. Steve Hsu where he answered this very question.

Apparently, when scientists do a Genome Wide Complex Trait Analysis (GCTA), which is a technique where a sample of people are assigned to pairs, and then the phenotype-genotype difference in all the pairs is correlated, they can determine the number of  single nucleotide polymorphisms (SNPs) that are associated with a one standard deviation difference in phenotype.  On the IQ scale, a one standard deviation difference is about 15 IQ points.  Once they know how many  SNPs are associated with a 15 point IQ difference, they square that number of SNPs to estimate the total number of cognitive variants.  So apparently, a difference of 100 SNPs has been associated with a 15 point IQ difference and since 1002 is 10,000, they estimate IQ is associated with 10,000 genetic variants.

Now you may be wondering why they square the number of variants that “cause” a one standard deviation IQ difference.  I assume it’s because the standard deviation squared is known as the variance, thus you have to square the number of SNPs that cause one SD change in IQ, to estimate how many cause the total variance in IQ.

Now I’m confused because in this source Hsu writes:

Average pairwise genetic distance changes with mean IQ and IQ difference: ∼ 39 SNPs per population SD

So in one source he’s saying that 100 SNPs are associated with a 1 SD change in IQ, but in another source he’s saying 39 SNPs.  Perhaps this is just an example of regression working both ways?  In other words people with a 100 SNP difference average a 15 IQ point difference, and people with a 15 IQ point difference average a 39 SNP difference.  Kind of like parents with an IQ of 125 average kids with an IQ of 115 but kids with an IQ of 115 average parents with an IQ of 109.

But this is where it gets really interesting.  Hsu writes:

Given that there are many thousands of potential positive variants, the implication is clear: If a human being could be engineered to have the positive version of each causal variant, they might exhibit cognitive ability which is roughly 100 standard deviations above average. This corresponds to more than 1,000 IQ points.

I don’t follow the math here.  If there are only 10,000 variants, and 100 variants causes a one SD increase in IQ,  then to be 100 standard deviations above average, wouldn’t you need 10,000 more positive variants than the average person?  But I thought there were only 10,000 variants in total so wouldn’t that imply the average person has no positive variants?