One of the arguments against allowing Eastern European immigration in the 1920's was that Eastern Europeans (mostly Jewish) were tested as having IQ's in this range. And most of the eugenics arguments about "breeding" poor people was made by English nobility against Irish immigrants in England. And we all know how stupid President Kennedy and President Reagan are. (Reagan's father was Irish Catholic).
And in Africa, many children are brain damaged from malnutrition. We saw a huge increase in academic ability in second generation African students: I.e. whose parents had education and jobs where they could feed children decently. South Africa and Rhodesia did some work in child nutrition, but as a whole, it was not a priority.
I could buy into that if it wasn't for the corresponding long-standing SAT/IQ Black/White gap here in the US. With obesity a highly visible problem in even poor areas, malnutition seems an unlikely cause. Particularly after read the part in "The Bell Curve" where the SAT scores of MIDDLE-CLASS ( $50K+ ) Blacks averaged lower than the SAT scores of poverty-range Whites.