Brazil's constitution (1988) granted municipalities the responsibility of providing social services. Many observers anticipated that this newfound authority would produce policy diversity, as local governments would tailor programs to constituents' needs. Instead, many municipalities chose to replicate programs made famous elsewhere. What explains this diffusion of social policies across Brazil? In particular, what motivates policy makers to emulate “innovative” policies? This study compares three approaches that seek to explain political behavior: political self-interest, ideology, and socialized norms. It draws on two policies, Bolsa Escola, an education program, and Programa Saúde da Família, a family health program, in four exemplary cities, to uncover the mechanisms that led to diffusion. Surprisingly, political incentives, such as electoral competition, cannot explain diffusion. Rather, ideology and socialized norms, transmitted through social networks, drive policy emulation. Diffusion occurs when politicians are ideologically compelled to replicate these programs and when policy specialists seek to demonstrate that they follow professional norms.