Nutrition is discussed in the link as a possible explanation.

In the context of this thread, though, I was replying to the question of whether simply pointing out the statistics might be seen as overt racism by outsiders. "Blacks are over all less intelligent and more prone to violent crime as compared to whites and Asians" sounds very much like what the educated nineteenth century racists said, the same way "I like eugenics" sounds like "I like genocide" to someone who could reasonably expect to be victimized by policies based on those ideas.

Fair enough.