Since mixed racial background should make a difference in genes but makes only a small difference in the way our culture treats a person, if the IQ gap is the result of genetics we should see find that the those with mixed race backgrounds have higher IQs than those of mostly or exclusively African descent. This has been approximated with skin tone studies in the past and my recollection is that one study showed a slight correlation between lighter skin tone and IQ and the other study showed no correlation. There just hasn't been much research done and I doubt there will ever be much research (which is fine by me).