Homicide rates have plummeted nationally since the mid-1990s. Even so, Portland’s homicide rate on a per capita basis has been among the nation’s lowest for years. And though academic criminologists offer a host of data-driven theories that they say correlate a variety of policies and demographics with murder rates, none appear to fully explain why in Portland murder rarely occurs.

“Almost every major city in the U.S., you can locate the areas of the city that have high murder rates by finding the neighborhoods that are impoverished,” says criminologist Jack Levin, who teaches courses on the sociology of violence at Northeastern University in Boston.

Cities with high rates of poverty have higher murder rates, Levin says.

Levin says 90 percent of the murders in Boston take place in three low-income neighborhoods. He did a comparison study between Boston and St. Louis, two cities similar in size. Boston had a murder rate of 11 per 100,000 residents, St. Louis’ rate was 46 per 100,000. Boston had 23 percent of its citizens living below the poverty level while St. Louis had 30 percent.

Another determining factor in homicides, according to Levin, is immigration — the more foreign-born residents a city has, the less likely it is to see murders.

That theory worked in Levin’s Boston/St. Louis comparison. Boston had many fewer homicides and 26 percent of its residents were foreign-born immigrants. St. Louis had more homicides and only 7 percent of its residents were immigrants.