A cultural explanation could exclude a genetic one. Simply put, the culture transmitted by black parents is not conducive to intellectual growth, just as the culture transmitted by Ashkenazi Jews is conducive to intellectual growth. This would also explain Alicorn's example, as the mother is more likely to do most of the cultural transmission, it would explain that data.

I'm not advocating this position, and I'm certainly not generalizing about every single member of a very large group, but this would explain the observed discrepancy and data without requiring a genetic basis. The actual explanation is doubtlessly more complicated; the point is that there are certainly other ways of explaining observed data that do not rely on genetics. That doesn't mean that genetics isn't a factor, only that it's not the case that it must be a significant one.

Also, while we're at it, I hate the term "significant." It's one of the most effective weasel words in existence.

If I wanted to claim that any one of these factors plays a significant role in the difference, I'd need to provide evidence. Because genetics is hard to see and so directly intertwined with other factors (the parents who create you generally raise you), claiming, "Genetics must be a key factor!" requires a significant amount of unambiguous evidence.

I admit there may be better evidence on this than I am familiar with, but I would be very surprised if that were the case. Good data on this topic is very hard to procure funding for.

I agree wholeheartedly with NT's statement, though. People unwilling to entertain the possibility that genetics differ between ethnic subgroups are indeed failing at rationality, though I'd have to say a socially motivated failing at rationality is less blameworthy than a personally motivated one.