Lena Edlund, a Columbia University economist, and Cecilia Machado, of the Getulio Vargas Foundation, lay out the data in a new National Bureau of Economic Research working paper. They estimate that the diffusion of phones could explain 19 to 29 percent of the decline in homicides seen from 1990 to 2000.

“The cellphones changed how drugs were dealt,” Edlund told me. In the ’80s, turf-based drug sales generated violence as gangs attacked and defended territory, and also allowed those who controlled the block to keep profits high.

The cellphone broke the link, the paper claims, between turf and selling drugs. “It’s not that people don’t sell or do drugs anymore,” Edlund explained to me, “but the relationship between that and violence is different.”