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data from ship records on the number of slaves shipped from each African port or region with data from a variety of historical documents that report the ethnic identities of slaves that were shipped from Africa.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.
In his working paper Nunn attributes these conditions to the collapse of states and the encouragement of ethnic feuding that was the result of raids, kidnapping and internal warfare (his explanation will presumably be longer and more detailed in the final paper.) Granted, correlation is not causation, but it's a sustainable (and widely held) proposition that protracted political and economic instability in Africa resulted from economic exploitation and the slave trade, and this study suggests that this conclusion is appropriate.
It's very easy for some people to dismiss the long-term effects of the slave trade had on Africa, as the slave trade ceased almost two hundred years ago and the economic exploitation of Africa ended well over a hundred years ago. But it bears reminding that the slave trade worked it's grievous toll on Africa over a period of nearly four hundred years, and it's effects can hardly be overcome in as little as four or five generations.
UPDATE: On the same topic, Mount Vernon has opened a new exhibit in the form of a log cabin designed to show how slaves owned by George Washington lived.