Places with more executions should in theory have more murders. They don’t.

In 2013, the Death Penalty Information Center found that just 2 percent of America’s counties carried out more than half of all executions in the US between 1976 and 2013. The findings were startling: Although 31 states still have the death penalty, the controversial policy is mostly used by just a fraction of counties.

Why do some US counties execute far more people than others in America? It’s not, it turns out, because they have more homicides. When Frank Baumgartner, a political scientist at the University of North Carolina Chapel Hill, looked into the question, he found little to no correlation between counties with high numbers of executions and high numbers of homicides.

Using data compiled by Baumgartner and student researcher Woody Gram, UNC associate professor Nikhil Kaza put together maps showing the lack of a correlation:

The map looks at the best available county-level data for both executions (based on where the trial occurred) and homicides, so the timespans analyzed don’t perfectly match due to missing data. But Baumgartner explained that he wouldn’t generally expect to see big differences based from the years that are missing — Baltimore consistently has more homicides than Des Moines, Detroit has more than Atlanta, and so on.

Also, for the states marked as not having capital punishment on the map, researchers based that label on whether states had the death penalty for most of the time periods analyzed. But over the past decade, several states — including New York and Illinois — have abolished the death penalty, even if they’re not marked as “states without capital punishment” on the map.

With those caveats in mind, the message from the map is pretty clear: There is little to no correlation between homicides and executions. And since the research and experts indicate that the death penalty is not a deterrent to crime, something else must be at play.

So what’s the actual explanation for higher rates of executions? Part of it is cultural norms and public support: Southern and western states hold more favorable views of the death penalty than other parts of the country.

But that doesn’t explain the entire difference. After all, there’s no reason to think that cultural and public values in southeast Texas — and Houston in particular — would be far more supportive of the death penalty than nearby Louisiana, Mississippi, Alabama, and Georgia.

Baumgartner, who’s researching this issue, put forward a different theory: Once a county executes someone, it becomes easier — even more encouraged — to do it again. For example, in counties that use the death penalty quite often, prosecutors may be rewarded for landing a death sentence — the strictest possible punishment — in a homicide case. “Let’s say you’re the assistant district attorney in San Antonio, Texas, and your colleague in the next room has had three executions to his so-called credit,” Baumgartner said. “Then you need to get yours to get promoted.”

It also might get easier to justify another execution after several of them. “If you’ve never had an execution and then the next terrible murder occurs, the prosecutor’s perspective might be, ‘Well, it probably wasn’t the most heinous murder ever in the history of our county, and therefore, by fairness, it doesn’t really require the death penalty,’” Baumgartner said. “On the other hand, if you’ve already had 20 executions in your district, and then there’s another horrific murder, then it really might be worse than some of the other murders for which people have been executed.”

What this really shows is the tremendous power of prosecutors, an often overlooked actor in the criminal justice system. Much of the public and media attention over the criminal justice system goes to lawmakers, police, and prisons. But it’s really prosecutors who carry most of the power: They decide how to interpret a law — whether, for example, to pursue murder or manslaughter charges in a case — and the sentence they want to pursue, up to and including life in prison and the death penalty.

For example, just one parish in Louisiana — Caddo Parish, which includes Shreveport — effectively kept the death penalty going in Louisiana throughout the past five years. As James Gill reported for the New Orleans Advocate, the two prosecutors in Caddo, Dale Cox — who once said it was a shame that executions weren’t more frequent — and Hugo Holland, landed six of eight death sentences handed down in the state in the last five years. (Cox resigned from the district attorney position earlier this year.)

So in the case of the few counties that execute a tremendous amount of people, the trend may fall entirely on one individual who leads a tremendously powerful, but very local, political office. At the very least, it doesn’t have much to do with homicide rates.