In other words, hygienic, well-fed life enables people to maximize their genetic potential so that the only variation left is innate. Intelligence becomes significantly more heritable when environmental hurdles to a child's development have been dismantled.
This effect has been shown conclusively for physical growth, both at the level of the individual and at the level of populations. You know how people tend to be taller than their parents? Interestingly, it's also been shown that each generation can only gain so much height --- gains are physiologically constrained at the population level.
People have examined linear growth in populations dating back to the early 19th century by using data from conscripts. They find that average height increases with each generation in societies that are relatively wealthy and healthy. Wars and famines cause drops that are then "caught up" when the problems end.
I suspect that eventually, this same general pattern will be found for cognitive ability. It's why I been known to rave occasionally about what I believe is a very important need to provide nutritious food and good healthcare to people of low SES.
