This seems to be a clear case of nurture and not nature influence in intellectual capabilities. A recent study by UC Berkeley's neurophysicists, on which I've read at Science Daily, reports that, all else equal, children growing in poor households have frontal lobes that are not really functional.
Kids from lower socioeconomic levels show brain physiology patterns similar to someone who actually had damage in the frontal lobe as an adult...
It's not just that these kids are poor and more likely to have health problems, but they might actually not be getting full brain development from the stressful and relatively impoverished environment associated with low socioeconomic status: fewer books, less reading, fewer games, fewer visits to museums.
These kids have no neural damage, no prenatal exposure to drugs and alcohol, no neurological damage. Yet, the prefrontal cortex is not functioning as efficiently as it should be. This difference may manifest itself in problem solving and school performance.