How can you even properly isolate that?
How do you know lower IQ comes from the genetics or come from shared environmental factors, like an alcoholic mom that drinks during pregnancy, environment contaminated with substances that are known to lower IQ like lead, malnutrition, etc.?
I linked the study, you can read for yourself, but the basic idea is comparing monozygotic twins that were adopted into different families at birth.
The only shared environmental factors that remain is the time in the womb.
Another thing they do is look at twins that were raised together, comparing monozygotic twins to dizygotic twins. The former have IQs that are closer to each other, again pointing to genetic factors. And they look at difference in IQ between non-genetic siblings (almost no relation).
Also, it might be interesting to note that besides IQ, a variety of complex behaviour is better predicted by looking if the birth parents did that behaviour than the adoptive parents.
We think monkey see monkey do, but in practise the data shows monkey is monkey do. At least for such behaviours as smoking, frequent drinking, and more.
They also look at the relationship between adopted parent (guardian) IQ and the kid and (not just during childhood, but up into middle age, the advantage of IQ testing is that we've had it a long time).
The relationship between guardian IQ and kid IQ is so small as to probably nonexistant. It goes against our intuition, but that's what the data shows.
Why doesn't everybody know this when that's what the data shows? I don't know, but I think there are political motives for that. After all, if the only thing seperating our differences is how we are educated/raised, then population replacements would be benign. Nevermind that the same people also push diversity because different people think differently. Political motives never aim for complete intellectual consistancy.
It also ties back into democracy in that it's only ever about competing interests instead of united interests when you have multiple demos, multiple peoples.