From Philip Gourevitch’s interview on Booknotes:

“Hutu and Tutsi, although they're called ethnic groups or tribes or races, they--nobody knows exactly how they came into being as social categories in Rwanda. The difference is that, for our--the entire colonial period, a very strict concept of Tutsis as a superior race and Hutus as an inferior race, this elite minority of Tutsis who lorded it over them--the rest and who essentially harnessed their labor and exploited them as a monarchist class, they became privileged and an almost apartheid-like system was imposed by the Belgians, with identity cards defining your ethnicity.“