It’s one of Mayor Rahm Emanuel’s favorite themes: The importance of summer jobs for at-risk youth.

But researcher Sara Heller, to her surprise, had found very little evidence backing up the idea that having a summer job actually lowers crime rates — until now.

In a study released this week in the online edition of Science Magazine, Heller found that disadvantaged youth accepted into an experimental city summer jobs program committed, on average, almost half as many violent crimes as those who applied but didn’t get into the program.

And those statistics held true some 16 months after the two-month jobs program ended, Heller found.

“It’s something that is exciting for the city,” said Heller, who got her doctorate at the University of Chicago and is now an assistant professor of criminology at the University of Pennsylvania. “We don’t have a lot of rigorous evidence on how to reduce youth violence, but it’s such a devastating social problem that everyone is desperate for evidence of what works.”

Heller’s study looked at an experimental program started in 2012 that targets at-risk city youth. That program is part of a larger jobs program involving about 22,500 youth, ages 14 to 24, last summer.

The study followed about 1,600 young people — ages 14 through 21 — who applied for the experimental program. Some 700 were picked through a lottery, with the other 900 turned away from that program, Heller said.

Jobs included everything from clerical work to being a day camp junior counselor. Each participant was also assigned a mentor.

With the help of Chicago Police, Heller tracked arrests for violent crime and found there were 43 percent fewer among those who had been accepted into the program.

“The bulk of the violence declined after the program [was] already over,” Heller said. “That’s telling us the program is changing future behavior, not just behavior over the summer.”

It’s unclear if those trends will continue. Heller said the study is ongoing.

Not surprisingly, folks at City Hall are delighted by the results. Commissioner Evelyn Diaz, whose Department of Family and Support Services oversees the summer jobs program, helped create the experimental program.

“The study is important to us because it really confirms our belief that we can reduce violence with a low-cost and short-term program, and it validates [why] the mayor has been putting tons of money into expanding summer jobs,” Diaz said. “It tells us that’s the right place to put our money.”

That said, the experimental program doesn’t come cheap. It costs the city about $1,000 for every youth who participates in the city’s traditional summer jobs program. It’s about $2,900 for every youth involved in the at-risk program.