They don't need to be, at sample sizes in the 100,000s. Most of these studies are not even using IQ tests but "years of education" which is a measure of IQ with a laughably large standard error - but with very large sample sizes very precise estimates can be made of the effect of a particular SNA on the affected people's intelligence.
Using "years of education" as a measure seems like it might just end up measuring how well you can use genes to identify whether someone's of the right socioeconomic class to get their children into university. I can't really see any way around this.
Well I believe they control for social status. And also a lot of other things, see gwern's comment above. In any case, I don't expect there to be genes that predict social class. And if there were, that would be super interesting in itself.
>I don't expect there to be genes that predict social class
Genes for skin pigment
>super interesting
If you've never heard of inequality before, I guess
But they control for race, and picked results that generalize across different populations.