I've read something similar somewhere, perhaps it was "Africa: History of a Continent," by Reader. I can't remember if that was the correct source. I think the thesis was that the loss of manpower and subsequent fragmentation and perversion of social relations led to a prolonged backwards mentality. African "nations" were slow to embace concepts of nation-states, and tribalism persisted. The latter comment doesn't necessarily have anything to do with the effects of slavery; but it is a particular phenomenon of many African areas.