Whatley and Gillezeau connect two strands of recent economic history: the claim that Africa experienced a reversal of fortune, and the claim that ethnic fragmentation helps explain the continent's underdevelopment. Their contribution is to trace present-day fragmentation back to the Atlantic slave trade.

The empirical case rests on both correlation and an instrumental-variables strategy meant to identify a causal effect. Measured against slave exports, ethnic heterogeneity in Africa today comes out higher where the trade drew more people away, and the estimated relationship is large enough to matter economically as well as being statistically firm.

The authors test the finding against the obvious objections, varying how ethnic boundaries are drawn and changing the unit of observation; the result holds up under both.