He earned his PhD from Harvard University in 1968 and also holds degrees from Syracuse University. He is a specialist in the study of ethnic conflict and author of the books Ethnic Groups in Conflict (University of California Press, 1985), A Democratic South Africa? Constitutional Engineering in a Divided Society (University of California Press, 1991) and The Deadly Ethnic Riot (University of California Press, 2001). He has acted as a consultant on the problems of divided societies and on policies to reduce ethnic conflict in locations including Russia, Romania, Nigeria, Tatarstan and Northern Ireland. In 2006, Horowitz was appointed to Secretary of State's Advisory Committee on Democracy Promotion.