This is well known to anyone familiar with twin studies. Sandra Scarr, after conducting the Minnesota Transracial Adoption Study: “Within the range of ‘humane environments,’variations in family socioeconomic characteristics and in child-rearing practices have little or no effect on IQ measured in adolescence.” P. 476
“There is simply no good evidence that social environmental factors have a large effect on IQ, particularly in adolescence and beyond, except in cases of extreme environmental deprivation.” P. 476
Recent study discussed in New Scientist:
It is clear that intelligence is at least partly genetically determined. This was supported by the discovery in 2001 that the volume of the brain’s grey matter, made up of “processor” cells, is heritable and correlates with certain elements of IQ (Nature Neuroscience, DOI: 10.1038/nn758). The amount of white matter, which provides the connections between these processors, has since been shown to be heritable too (Journal of Neuroscience, vol 26, p 10235).
Now it seems that the quality of these connections, which is governed by the integrity of the protective myelin sheath that encases them, is also largely genetic, and correlates with IQ.
Paul Thompson and colleagues at the University of California, Los Angeles, scanned the brains of 23 sets of identical twins and the same number of fraternal twins, using a type of magnetic resonance imaging called HARDI. MRI scans typically show the volumes of different tissues in the brain by measuring the amount of water present. HARDI measures the amount of water that is diffusing through white matter, a measure of the integrity of myelin sheathing, and therefore the speed of nerve impulses. “It’s like a picture of your mental speed,” says Thompson.
http://www.newscientist.com/article/mg20126993.300-highspeed-brains-are-in-the-genes.html