IQ correlates most strongly with socioeconomic class, with members of the same ethnic group scoring higher over the decades as that ethnic group as a whole becomes wealthier.
Socioeconomic status limits genetic potential. Thus the effects of SES dominate for those in poverty, but heritability dominates for those with higher SES.
For the intuition think of height - a malnourished child will not reach their “genetic” height. A fully nourished child will be limited by their “genetics”. Why wouldn’t other biological characteristics be similar?
Exactly the opposite is true. Adoption studies have been used to isolate the effect of SES itself, and the contribution of that factor is low: https://www.sciencedirect.com/science/article/abs/pii/S01602... (“Proportion of variance in IQ attributable to environmentally mediated effects of parental IQs was estimated at .01… Heritability was estimated to be 0.42.”).
This is just SIBS data. It has all the standard "Minnesota" limitations: the study is tiny, the cohort isn't demographically representative, adoption isn't itself random, nothing deconfounds the prenatal environment, and the children in the cohort are also adopted at different ages.
It's one thing to call out an interesting paper; it's another to act as if the matter has been settled simply by pointing to SIBS.
For the intuition think of height - a malnourished child will not reach their “genetic” height. A fully nourished child will be limited by their “genetics”. Why wouldn’t other biological characteristics be similar?
https://en.wikipedia.org/wiki/Scarr%E2%80%93Rowe_effect