We need to be careful with blaming race/genetics. Poverty and correlating factors like malnutrition, repeat bouts with severe illness (malaria) and parasitic infection all contribute to lower intelligence.
How a Worm Gave the South a Bad Name
Hookworms once sapped the American South of its health, yet few realize that they continue to afflict millions.
http://www.pbs.org/wgbh/nova/next/nature/how-a-worm-gave-the-south-a-bad-name/
A 1926 study of Alabama school children found that the greater the number of worms that students harbored, the lower their IQ.