Commenter Swank posted a link that lead me to an excellent large-scale study where Swedish young men (ages 18-20) were adopted and compared to their same age genetic relatives who grew up in different homes. What the study found was that being raised by educated parents increases IQ, even in young adulthood, even when you control for genetic background.
The study ranked the education of the parents on a scale of 1 to 5. I’m not sure how this scale worked because I don’t have access to the full study, but you can imagine how such a scale might look in the United States:
Level 5 = Advanced university degree
Level 4 = University degree
Level 3 = High school diploma
Level 2 = 8th grade education
Level 1 = Less than an 8th grade education
The study found that each additional level of parental education added about 1.83 IQ points (1.71 to 1.94) to the young man’s IQ (controlling for the young man’s genetics).
What this study seems to suggest is that if you have a pair of identical twins, and one of them is raised by a seventh grade dropout (level 1 education?) and the other is raised by a PhD (level 5 education?), then by the time they are 19, the one raised by the PhD should score 7 IQ points higher.
Does this mean we should start subtracting IQ points from people raised in educated homes and adding IQ points to people raised in uneducated homes? Probably not because most people are not adopted, and so the extra IQ points caused by their educated parents correlate with the high IQ genes they inherited from their educated parents. So while differences in parental education widens the absolute gap between people raised in different homes, they don’t much change the rank order for non-adopted people, and IQ is a measure of rank order, not absolute differences.
It’s worth noting that the vast majority of Americans are not raised by PhDs or seventh grade dropouts, so that’s quite an extreme case. Randomly selected Americans would not differ anywhere near that much in parental education levels, so such differences would not significantly contribute to IQ variation, except perhaps on tests like the SAT, where socioeconomic effects are suspected to be more pronounced (though this is controversial).
Although being raised by more educated parents props up IQ, that’s not the same as saying it props up intelligence. I suspect being raised by educated parents exposes kids to more cultural experiences, which increases their performance on measures of knowledge and vocabulary. Educated parents probably also encourage their kids to stay in school longer, and train them to value intellectual tasks like sitting still and concentrating, and such attitudes can add a few extra points on IQ tests, but they don’t reflect genuine gains in intelligence. Further, these effects are probably large in childhood, but become small in late adolescence as this study shows. I suspect if there were a followup study at age 40, the effect would have all but vanished.
Only when environment increases the biological environment, particularly in the prenatal stage, do the IQ gains tend to reflect a genuine rise in intelligence. This is because intelligence appears to be an overwhelmingly physiological variable, that is not amenable, to any significant degree, to psychological intervention, except perhaps in the most extreme of cases. That doesn’t mean the way you raise your kids is irrelevant, but it’s perhaps, largely irrelevant to intelligence.