I'm reluctant to get involved in the politically incorrect side of this argument, but FWIW, that stat comes from "Intelligence: Knowns and Unknowns", which was a paper published in the mid-90s by the American Psychological Association (in response to the controversial book "The Bell Curve").
From the paper:
"The differential between the mean intelligence test scores of Blacks and Whites (about one standard deviation, although it may be diminishing) does not result from any obvious biases in test construction and administration, nor does it simply reflect differences in socio-economic status. Explanations based on factors of caste and culture may be appropriate, but so far have little direct empirical support. There is certainly no such support for a genetic interpretation. At present, no one knows what causes this differential."
