"Some cities are experiencing gang problems that are spiraling out of control, and others are not," says Professor Fox. "That's essentially the issue."
Criminologists cite a variety of factors for the increases, from a drop in the number of officers on the beat to a shift in resources to fight terror to cuts in federal spending on youth programs and gang prevention. Alfred Blumstein, a criminologist at Carnegie Mellon University in Pittsburgh, also cites a lack of economic opportunity for young people.
"The phenomena are very different and local across the country: There's now no major national drug epidemic like there was in the early 1990s," says Professor Blumstein. "All politics is local. Also, these days, all crime is local, too."
Still, it's clear that the availability of resources for police and prevention programs – as well as economic development – plays a key role in determining which cities have higher homicide rates. Take New York, at its 40-year low, and compare it to Baltimore, which is at an eight-year high.
"New York has more money for youth programs, prevention programs, and things like summer jobs, so they're able to control their gang problem," says Fox. "Whereas other cities like Baltimore don't have the resources to offer the same alternatives."