However, it’s important to realize that environment is generally fairly constrained in these studies. A Twin study is hardly comparing Sierra Leone with Japan, or the Netherlands with North Korea. Extremes like iodine deficiency, disease, or fetal alcohol syndrome are similarly excluded.
except if you look at the other comparisons made (eg. "Unrelated children—Reared together—Adults "), the correlation factor can go quite a bit lower than 0.7-0.8 (eg. 0.04). The fact the correlation can go this low casts doubt on your theory that the environment is "constrained".
“Same person (tested twice) .95 Identical twins—Reared together .86 Fraternal twins—Reared together .55 Biological siblings—Reared together .47 Unrelated children—Reared together .30 Parent-child—Living together .42 Adoptive parent – child – Living together 0.19 Identical twins—Reared apart .76 Fraternal twins—Reared apart .35 Biological siblings—Reared apart .24”
So siblings reared apart had less correlation than unrelated children living together.
Like what “raised separate” means.
Was one raised in a Soviet gulag while the other was raised in an Israeli kibbutz?
That makes me think it's just a generic "I don't like this conclusion so I'll criticize it" Joker card that people play, and not a legitimate debunking.
Sure. To take this line of reasoning to its conclusion, an environment that has no oxygen or is otherwise uninhabitable, would prevent any human from living or having any mental abilities whatsoever. In this sense, environmental influences would completely dominate genetics. Similarly, changing 50% of a human's DNA would have them live life as a stalk of wheat.
What's your point?
FWIW, you see the similar trend in adopted children - as they grow older, their IQ trends from being close to their adoptive parents to getting closer to their (completely uninvolved) biological parent.