Re processing of DNA etc, this isn't the level these studies are typically working at. There's no biochemistry knowledge involved. They are just looking at DNA

Re processing of DNA etc, this isn't the level these studies are typically working at. There's no biochemistry knowledge involved. They are just looking at DNA sequences, and what data they have about outcomes. Whether a piece of DNA makes something critical for some muscle, or is 10 steps away in part of some complicated switching apparatus, of indeed just influences something else which causes the growing animal to enjoy behaviour which happen to exercise this muscle -- all of this detail is a black box, all of these would contribute to genetic effect. Re universal education, etc: in adoption studies this translates to "among parents who will be approved to adopt", and this indeed cuts the very bottom off the distribution of some factors. I don't think it's a huge issue, but not an expert. But my understanding of the point Plomin is making there is a bit different. Regardless of the current state of (say) schooling, he's pointing out that moving from a system with large schooling inequalities, to a system of absolute schooling equality, would almost certainly increase the heritability of education. In the unequal system, some bright kids are stunted etc, weakening the correlation between input and output; but in the equal one, all the variation in the output will be driven by the input. At least I think that's what he's saying. Haven't read the book, and not really my field at all...