The Arab Slave Trade and the Diffusion of Islam in Africa

Abstract

This paper investigates the role of the Arab slave trades in the diffusion of Is- lam in Africa. Islamic law prohibited the enslavement of free Muslims, making conversion a collective strategy to reduce raiding risk. Using newly digitized historical sources linked to contemporary data on religious affiliation and at- titudes, we show that groups with greater exposure to Arab slaving routes exhibit a higher historical prevalence of Islam, as measured by the presence of mosques, and a higher contemporary share of Muslims. They also report stronger religiosity and more conservative political and gender norms. Results are robust to rich geographic and ethnographic controls, to using propensity- score matching and neighboring-group fixed effects, and to excluding areas historically under Muslim rule. We do not find any effect when analyzing the Atlantic route, run by Europeans, and where conversion offered no legal protection. Three additional patterns support the hypothesis of preventive conversion. First, effects dilute once an area has been exposed to raiding, in line with raiders not being able to enslave more individuals and redirecting toward non-Muslim frontiers. We do not find similar timing pattern in the At- lantic route. Second, effects are weaker in rugged terrain that impedes raids, where conversion incentives were therefore lower. Third, we find spillovers in areas not directly raided but adjacent to exposed groups, and these spillovers are stronger where historical cross-boundary mobility was easier.