Why would a government introduce a strong external oversight mechanism over its own administration? Independent Police Oversight Agencies (POAs) have been established to better protect human rights. Using a novel data set of 27 POAs, we examine two dimensions of their strength: de jure and de facto independence. Their best predictors are the de jure independence of judges or prosecutors, the budget of the judicial system, and the push by the international environment. This lends support to a “genetic-and-diffusion” model for explaining the strength of POAs combining two logics (genetic/culture and diffusional/tactical) operating at two levels (national and international).