Via Dani Rodrik, a new paper by Nathan Nunn finds that the slave trade had very severe, very horrific, and very long-lasting effects on Africa's development. Okay, that's no surprise, but it's illuminating to read Nunn's account of exactly how the slave trade ravaged certain nations and regions:

The most common manner in which slaves were taken was through villages or states raiding one another. Where groups of villages had previously developed into larger scale village federations, relations between the villages tended to turn hostile. As a result, ties between villages were weakened, which in turn impeded the formation of larger communities and broader ethnic identities. ... Because of this process, the slave trades may be an important factor explaining Africa's high level of ethnic fractionalization today. ...

Because of the environment of uncertainty and insecurity at the time, individuals required weapons, such as iron knives, spears, swords or firearms, to defend themselves. These weapons could be obtained from Europeans in exchange for slaves, which were often obtained through local kidnappings. This further perpetuated the slave trade and the insecurity that it caused, which in turn further increased the need to enslave others to protect oneself. ...

Generally, the consequence of internal conflict was increased political instability and in many cases the collapse of pre-existing forms of government. In 16th century Northern Senegambia, the Portuguese slave trade was a key factor leading to the eventual disintegration of the Joloff Confederation, which was replaced by the much smaller kingdoms of Waalo, Ka joor, Baol, Siin and Saalum. Further south, in Southern Senegambia, the same pattern is observed. Prior to the slave trades, complex state systems were in the process of evolving. However, this evolution stagnated soon after the arrival of the Portuguese in the 15th century. ...

Pre-existing governance structures were generally replaced by small bands of slave raiders, controlled by an established ruler or warlord. However, these bands were generally unable to develop into large, stable states. Colson writes that "both the bands and the new states they created retained an air of improvisation. Few band leaders were able to hand power to a legitimate successor. Even where a band leader had become the ruler of a state, succession remained a problem. Leadership was a personal role, rather than an established office."

The slave trades also contributed to political instability by causing the corruption of previous established legal structures. In many cases, it became common to obtain slaves by falsely accusing others of witchcraft or other crimes. Klein writes that "communities began enslaving their own. Judicial penalties that formerly had taken the form of beatings, payment of compensation or exile, for example, were now converted to enslavement." Often, leaders themselves supported or even instigated this abuse of the judicial system. To protect themselves and their community from being raided, leaders often chose to pay slaves as tribute, which were often obtained through the judicial system.***

According to Nunn, the economic evidence bears this account out: The slave trade ripped apart many relatively stable states, undermined existing legal structures, and fomented ethnic factionalization. As a rule, the more a country was devastated by the slave trade, the worse its economic performance is likely to be today. Nunn also tries to rule out reverse-causation, noting that most slaves were taken from the better-developed parts of Africa, since it was easier for Europeans to cull slaves from countries with centralized governments and well-developed trading networks. (By contrast, Gabon, a poor and relatively violent part of Africa back then, scared off Portuguese traders.)

In any case, the paper appears to be well done, pinning down what many people had suspected for quite some time. Nunn also points out, surprisingly, that most of the impact of the slave trades were felt after colonial independence—since this is when Africa's pre-colonial political structures, which had been destroyed by the slave trade, suddenly became much more critical.

------------------
*** Here's one particularly gruesome example: "The chief of the Cassanga [in what is now Guinea Bissau] used the 'red water ordeal' to procure slaves and their possessions. Those accused of a crime were forced to drink a poisonous red liquid. If they vomited, then they were judged to be guilty. If they did not vomit, they were deemed not guilty. However, for those that did not vomit this usually brought death by poisoning. Their possessions were then seized and their family members were sold into slavery."