Nathan Nunn finds that "without the slave trades, 72% of Africa’s income gap with the rest of the world would not exist today":
The Historical Origins of Africa’s Underdevelopment, by Nathan Nunn, Vox EU: Africa’s poor economic performance is one of the largest puzzles in growth and development economics. A large literature has emerged trying to explain the source of Africa’s growth tragedy. See for example Easterly and Levine (1997), or Sachs and Warner (1997).
African historians have documented the detrimental effects that the slave trades had on the institutions and structures of African societies. Historical evidence from case studies show how the slave trade caused political instability, weakened states, promoted political and social fragmentation, and resulted in a deterioration of domestic legal institutions.
Between 1400 and 1900, the African continent experienced four simultaneous slave trades. The largest and most well-known is the trans-Atlantic slave trade where, beginning in the fifteenth century, slaves were shipped from West Africa, West Central Africa, and Eastern Africa to the European colonies in the New World. The three other slave trades -- the trans-Saharan, Red Sea, and Indian Ocean slave trades -- are much older and predate the trans-Atlantic slave trade. During the trans-Saharan slave trade, slaves were taken from south of the Saharan desert and shipped to Northern Africa. In the Red Sea slave trade, slaves were taken from inland of the Red Sea and shipped to the Middle East and India. In the Indian Ocean slave trade, slaves were taken from Eastern Africa and shipped either to the Middle East, India or to plantation islands in the Indian Ocean.
In a recent paper, I explore empirically whether these detrimental effects of the slave trades can explain part of Africa's current underdevelopment.