Would it follow, then, if we are adherents to the notion that a poor environment can permanently make one less intelligent, that a good environment can make a child who is bright into a gifted person (i.e. raise IQ to the gifted level when it was not otherwise destined to be there)?
Yes; these are just two ways of saying the same thing.
I don't think so. It's definitely true that an inadequate environment can permanently impair the cognitive ability of children; see
feral children. However, it could be that the impact of an adequate-to-good environment on intelligence sees a plateau. I believe that that's fairly well supported by twin studies, and in any event it's plausible. Poor nutrition for example can stunt brain growth, but past a certain point extra food and vitamins just make one fat and sick.
What I think is unaddressed by all the twin studies is the possibility that an extra-stimulating environment, not just a "good" one of the type normally studied, might encourage a child to grow more than normal. IIRC Turkheimer et al. have done some research on high ability people influencing their own environments, which might be responsible for long-lasting gains. Stimulating environments have been shown, I believe, to have a substantial impact on intelligence, but it can be mostly short-lived with a slight residuum. If people can self-stimulate, though, the sort of mindset that sets a genius apart might keep such a person nearly constantly peaking.
ETA: I think some fertile ground for exploration might be the identical twins whose IQs are substantially different, but who both came from good-or-better environments. Are there any where the intelligence differential is due to
educational environmental causes?
