As we all know from reading the newspapers, the reason for The Gap in school achievement, as proven by Hart & Risley,(see below) is because African-Americans don't talk enough.
But wouldn't this actually apply to New World Indians instead of blacks?
There is, however, no doubt that the various races, when carefully compared and measured, differ much from each other, as in the texture of the hair, the relative proportions of all parts of the body, the capacity of the lungs, the form and capacity of the skull, and even in the convolutions of the brain. But it would be an endless task to specify the numerous points of difference. The races differ also in constitution, in acclimatisation and in liability to certain diseases. Their mental characteristics are likewise very distinct; chiefly as it would appear in their emotional, but partly in their intellectual faculties. Everyone who has had the opportunity of comparison must have been struck with the contrast between the taciturn, even morose, aborigines of S. America and the lighthearted, talkative negroes.