He studies "criminal conflict"—organized armed violence involving non-state actors who, unlike revolutionary insurgents, are not trying to topple the state. His book Making Peace in Drug Wars: Crackdowns and Cartels in Latin America, forthcoming in the Cambridge Studies in Comparative Politics Series, examines armed conflict between drug cartels and the state in Colombia, Mexico, and Brazil. Lessing is currently conducting fieldwork for his second book, about prison gangs' takeover of urban peripheries in Brazil, El Salvador, and the United States, with support from the Harry Frank Guggenheim Foundation. He has also studied the effects of militarized policing and paramilitary groups' use territorial control to influence electoral outcomes.