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.
Ah, two different things going on. It's certainly true that there might be such threshold effects (and more generally, that the relative contributions of genetics and environment to observed phenotype might not be uniform across the scale of the latter) and I was eliding that without comment; sorry. My point was to consider a single fixed point of the phenotype at which you know there is some environmental contribution. If you know that two genetically identical individuals can end up with measured IQs of 129 and 130 because of differences in environment, then you can phrase this either as a good environment raising IQ or as a poor one lowering it. Without some external notion of which environment is "normal" there is no difference.
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 environmental causes?
It has taken me a while to understand what you're getting at here so bear with me if I'm still not getting it. The obvious answer to your last question is "yes, all". Identical twins are by definition genetically identical, so any difference between them is environmental; that's the point of such studies. Is your point that it would be interesting to identify people for whom there is evidence that they have
especially good environments - environments that are so rare today that their relative absence causes us to underestimate the effect of environment, because studies just don't contain enough people with environments that good - and see what they have in common? I agree that is interesting.
Indeed, the Flynn effect has to be caused by a general shift in what is regarded as a "normal" environment (because change has been too rapid for genetic change to contribute significantly), even if we don't know for sure what it is about the environment that has had this effect. Flynn's book makes this point, I think, that genetics/environment studies are only ever studying the range of environments that are actually around at the time of the study. One can fantasise about a researcher in 1912 identifying a few people who somehow had the 2012-normal environmental factors and studying them; assuming (dodgy I know) that the Flynn effect continues and will continue for another 100 years, we could be thinking about identifying the people today who have environmental factors that will be normal in 2112... It's a fantasy, because many of these factors are not, probably, things that can apply to a few individuals (e.g. in 1912 you might have had to find a few individuals for whom the integrated circuit had already been invented!) but an interesting one to me.
The issue of people altering their own environment, causing a feedback effect, is the main point of Gladwell's book Outliers as you probably know. Incidentally, it is the to-me-obvious fact that this also applies across the generations (parents who are successful, for whatever reason, typically give their children environmental factors that will increase their chances of being successful) that make naive regression-to-the-mean models such as the one in a certain recent thread annoying!
