From Dani Rodrik:
The slave trade, whereby able-bodied Africans were shipped to other parts of the world and sold into slavery, was a despicable economic institution for sure. But did it also have long-run effects on the economic development of African countries? Yes, is the surprising answer of Nathan Nunn (pdf link):
I construct measures of the number of slaves exported from each country in Africa, in each century between 1400 and 1900. The estimates are constructed by combining data from ship records on the number of slaves shipped from each African port or region with data from a variety of historical documents that report the ethnic identities of slaves that were shipped from Africa. I find a robust negative relationship between the number of slaves exported from each country and subsequent economic performance. The African countries that are the poorest today are the ones from which the most slaves were taken.
Nunn is careful to try to rule out reverse causation: he finds that the regions from which slaves were taken were, if anything, the more developed parts of Africa at the time.
The most likely explanation for the result? “[The] procurement of slaves through internal warfare, raiding, and kidnapping resulted in subsequent state collapse and ethnic fractionalization.”
There’s some interesting discussion in the comments there, too.