Jonah Lehrer writing in the Wall Street Journal talks about a study of twins showing that parental actions and choices have more of an influence on a child's early intelligence than genetic factors in poor families. As you move up the socioeconomic ladder, the reverse is true: genetic factors have more of an influence. Twins were studied at age 10 months and again at two years, long before they show up in kindergarten.
When it came to the mental ability of 10-month-olds, the home environment was the key variable, across every socioeconomic class. But results for the 2-year-olds were dramatically different. In children from poorer households, the choices of parents still mattered. In fact, the researchers estimated that the home environment accounted for approximately 80% of the individual variance in mental ability among poor 2-year-olds. The effect of genetics was negligible.
The opposite pattern appeared in 2-year-olds from wealthy households. For these kids, genetics primarily determined performance, accounting for nearly 50% of all variation in mental ability. (The scientists made this conclusion based on the fact that identical twins performed much more similarly than fraternal twins.) The home environment was a distant second. For parents, the correlation appears to be clear: As wealth increases, the choices of adults play a much smaller role in determining the mental ability of their children.
One telling analogy for this phenomenon is the genetics of height. It's long been recognized that genes play a much larger role in shaping the height of individuals in developed nations. This is largely because people from developing countries can suffer from inadequate nutrition, which stunts their physical potential. In contrast, the dietary variations among Americans are mostly insignificant – getting enough calories isn't our problem – which is why genetics has become the crucial factor. In any case, I think this research is a sobering reminder that the contingencies of childhood (such as the income and education of our parents) play significant role from the earliest days of childhood onwards. Life just ain't fair.