Gangs govern millions worldwide. Why rule? And how do they respond to states? Many argue that criminal rule provides protection when states do not, and that increasing state services could crowd gangs out. We began by interviewing leaders from 30 criminal groups in Medellin. The conventional...

Medellin's government wanted to raise its efficacy, legitimacy, and control. The city identified 80 neighborhoods with weak state presence and competing armed actors. In half, they increased non-police street presence tenfold for two years, offering social services and dispute resolution. In...