Large IQ differences among siblings in turn produce large differences among them in school achievement and life outcomes. Those differences, in fact, are almost as large as those found between strangers whose IQs differ to the same degree. Charles Murray recently quantified this in an ingenious study of pairs of American siblings raised together in non-poor homes. I will call this group the normals. These constraints produced a sample of 710 pairs.
Earned income is a good place to begin.
In 1993, when we took our most recent look at them, members of the sample were aged 28-36. The normals were in the middle, averaging 16,800. By the way, these earnings gaps are likely to widen with age, as the blue-collar workers' bodies wear out and therefore their incomes stagnate or fall. Within families, parents do a better job of equalizing children's environments than any government less tyrannical than the Khmer Rouge could accomplish. Yet, even with the same upbringing, IQ differences are both substantial and play a huge role in the kids' prosperity as adults.