From Brad DeLong:
If inherited genetically-based IQ were the source of the extra edge that the children of the rich get in our society, than we would expect a parent with 4 times average lifetime full-time earnings–say $200,000 a year–to have a kid with a lifetime average income of $51,500 instead of the average of $50,000. But it is not $51,500. It is $150,000.
Our obsession with the IQ test seems to exploit what I’ll call the quantification bias, which is the fact that being able to quantify something makes it seem more important than it really is. And so we fixate on IQ scores in such a wide variety of debates, from race to inequality, because it’s such a neat numerical score, even though it’s relevance to these debates (as DeLong points out) is not always clear. But it’s easier to argue about a quantifiable variable (that’s partly innate) than it is to focus on the real sources of inequality, like substandard schools.