He makes note that some studies indicate that at the similar income levels there is a narrowing of the gap. But the gap is well documented. There is a huge academic literature on the gaps in cognitive test results, practically all of it converging on the fact that African American mean scores on cognitive tests fall below the white means by a tad more than one white standard deviation. There is in fact so much data on this now that we have meta-studies — studies of the studies: the one best-known to me is the meta-study by Roth et al. in 2001, which covered 39 studies involving nearly six million test-takers. That one standard deviation on cognitive testing has been so persistent across so many decades, an academic sociologist, calls it "the fundamental constant of American sociology" — it's like the speed of light in physics .
http://www.amazon.com/The-Controversy-Media-Public-policy/dp/0887388396/
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