As I said, I’m not at all convinced that there is a genetic component to the black–white IQ gap. There is a stark environmental gap between blacks and whites in the U.S. — blacks have nearly triple the poverty rate of whites, and even blacks who aren’t poor are more likely than similarly situated whites to live in poor neighborhoods. As McWhorter writes, there are important cultural differences as well, regardless of class. I see no reason it can’t be the case that these environmental and cultural differences cause the entirety of the observed IQ gap.
I’m doubtful, however, that we will find no genetic group differences in any important trait whatsoever. There are countless social patterns besides the black–white IQ gap that could conceivably have genetic causes. East Asians and Ashkenazi Jews have higher average IQ scores than gentile whites. There are different levels of violence in different racial groups. Some societies have trouble setting up functional economies, a problem that seems to have “deep roots.”
Looking at the possibilities from a different angle, humanity spread out to every corner of the globe over the course of tens of thousands of years, facing radically different environmental challenges everywhere it went and evolving in lots of disparate ways when it came to skin color, bone structure, disease risks (including alcoholism), etc. In those environments humans set up radically different cultures as well, which can themselves drive genetic changes. What’s the chance that mental abilities and behavioral tendencies were completely unaffected by all these processes?