There is an overwhelming amount of evidence that the European empires did very little right in Africa. I will admit that these results came as a surprise to me; my prior would have been that the Europeans would have left behind some institutional innovations or physical infrastructure of value. That case, however, turns out to be extremely hard to demonstrate outside of Southern Africa.
The slave trade, of course, was one of the most egregious ways in which the Europeans managed to play havoc with Africa. It follows, of course, that ending it should have positive effects. And it did! The Middle Passage was a horrifying meatgrinder ... no possible cost could outweigh the reduction in human suffering from insuring that fewer people would suffer in it.
But the passage of the U.K. Slave Trade Act of 1807 may have had the consequence of boosting conflict in Africa. James Fenske (Oxford) amd Namrata Kala (Yale) argue that it did, by weakening the authorities whom had enriched themselves off slave exports. They take advantage of the fact that the British were rather more successful in reducing human trafficking from West Africa than elsewhere as a form of natural experiment to test their hypothesis.
The time series evidence is fairly compelling that something happened around then:
But I am not wholly convinced of the validity of their experiment. First, look at the above figure: there are a disconcerting number of zeroes among the controls. Second, look at the map to the right, which shows the treatment area in red and the control in blue. Are they really comparable regions?
The authors do vary the treated area and find the same result. In the map, the treated area consists of everything within 1,000 kilometers of a slave-trading port. They varied that distance to as low as 250 kilometers, with the same result. They also defined the treated zone as the current borders of countries with historical slave ports; again, similar results. Finally, they threw out the four territories annexed by European powers in 1807-40: Sierra Leone, Gambia, and Côte d’Ivoire. They also threw out Liberia for similar reasons. Again, the results hold.
Nonetheless, it still seems as though there may be a data problem: we know less about conflicts in the interior. And even if we did have comparable information, the interior likely had a radically different state structure and a much lower population density. The comparison seems forced.
Even without the experiment, the time series certainly make it seem like something happened to increase conflict in coastal Africa in 1807! The Slave Trade Act is the most probably culprit. Moreover, it is hard to argue reverse causality, i.e., that the Act was a response to rising levels of intra-African conflict. They do a good job of demonstrating that British interest in the slave trade had no correlation with interest in African conflict. (Google Ngrams is pretty neat.) In short, the experiment may be unconvincing ... but it also may be unnecessary. Read the paper!
But if they are right, it is depressing to think that ending the slave trade may have done as much to weaken African states as the trade itself.