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Coel Hellier's avatar

It seems to me clear that the heritability estimates from twin studies are correct, and the estimates from GWAS molecular genetics much too low. Here's the argument:

1) Twin studies measure the effect of the whole genome at once. That means that a sample size of ~ 50 will give a fair result.

2) GWAS studies are attempting to identify each genetic variation that contributes to differences in intelligence, and thence measure the effect of each genetic variation, and add these up.

3) Intelligence is a hugely complex trait; necessarily, any recipe for intelligence thus has to include a huge amount of information. That can only be if it involves a huge number of genes. You can't encode large amounts of information in a small number of genes.

4) Hence, variations in intelligence likely involve thousands of genetic variations (let's say 3000) each having (on average) a ~ 1/3000th effect on the overall difference.

5) The sample size needed to detect small signals scales as the square of how small the signal is.

6) Hence, for GWAS studies to find all of the genetic contributions to intelligence you'd need a sample size of ~ 50 * (1/3000)^2. That is 500 million. That is, you'd need to sample 500 million people's genomes. Obviously that hasn't been done.

7) If your GWAS study samples 1000 genomes then you can only find genes that contribute more than sqrt(1/1000th) of the variation. That is, the genes making ~ 3% contributions (actually. it's worse than that, for decent accuracy you need to repeat ~50 times, so you need a much bigger sample)

8) And that's exactly what they find, a handful of genes that affect intelligence at the ~ 3% level.

9) And that would be fine, if intelligence were indeed a trait controlled by ~ 30 genes each having a ~3% effect. But it can't be. There's no way one can encode the necessary oodles of information in 30 genes or even 300. Necessarily, it's going to involve 1000s of genes.

10) And that's why GWAS estimates are way lower; their sample sizes are only big enough to find the tip of the iceberg -- the small number of genes that each have a substantial effect -- but completely miss the thousands of genes that each contribute a small amount.

Refutations welcome; am I missing something?

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Billy's avatar

Great article, your response to Mr. Gusev was quite thorough. Nevertheless this quote that you stated in relation to the “missing heritability” is the main point of contention where we “shouldn’t shift our priors nearly as much as Gusev claims.” I’m not sure the response you laid out is convincing enough to not shift our priors much more. In regard to this, I believe we should at the very least be much more cautious and have a much more probabilistic approach as it relates to the estimates of heritability. There still seems to be much we don’t understand in that regards it seems!

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