Nathan Nunn,"The Long-Term Effects of Africa's Slave Trades," National Bureau of Economic Research, September:

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data from ship records on the number of slaves shipped from each African port or region with data from a variety of historical documents that report the ethnic identities of slaves that were shipped from Africa.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's explanation is that"procurement of slaves through internal warfare, raiding, and kidnapping resulted in subsequent state collapse and ethnic fractionalization." Andrew Sullivan picked up this report today. See also the discussion at Ampersand and Dani Rodrik's Weblog. Tim Burke and Jonathan Reynolds would have a better sense of things, but the conclusion strikes me as less surprising than some of the comments suggest. Nunn's data may simply confirm what we already knew.