Ironically, the very low average IQs found in Africa can't all be genetic in origin because the gap between Africans (mean IQ of 70 according to dozens of studies) and African-Americans (mean IQ of 85 according to hundreds of studies) is as large as the gap between African-Americans and white Americans (100). Yet, African-Americans are no more than 20% white by genealogy.
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Well nutrition (mentioned in the link) could be a factor, sure. But 20% (or more) White DNA would certainly raise the IQ of blacks by several points.
As for the rest?
Survival of the fittest....maybe....possibly.....perhaps.....
We know that the 70 IQ is an average and that some Africans have higher IQ's.
No doubt the strenuous pressures and struggle-for-survival circumstances that the slave trade created would've weeded out many of the less intelligent leaving a greater percentage of the smarter ones around. Throw in a generous helping of European DNA and a 14 point jump on the IQ meter doesn't seem so mysterious anymore.
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