It's ironic that a black man would use skull size to try to "prove" that women are less intelligent than men.
I guess he doesn't know his history very well as this was an 18th century attempt to justify slavery and other means of exploiting people of color:
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I suppose in Herd's case, his lack of intelligence has nothing to do with head size--his is quite large--but rather from a lack of appropriate education, training and subsequent disuse.