WASHINGTON — More states are getting rid of laws that automatically bump teenagers from juvenile courts when they reach a certain age, abandoning a model of punishment proven to be expensive, ineffective and not flexible enough to improve outcomes for offenders or society, a new study says.

The Justice Policy Institute focused on national trends and on what it called positive results in states that have most vigorously adopted policies designed to help teenagers and not send them to adult prisons. Research showed that states that have raised the age of jurisdiction — meaning the point where a teenager can no longer access the juvenile system and must be treated as an adult, no matter the crime — have seen decreases in crime.

“There are multiple reasons why places that raised the age avoided the dire predictions that juvenile justice systems would be overwhelmed, and why places considering raise the age proposals can pass them this year, knowing the change in jurisdiction can be managed effectively,” the study’s authors wrote.

States that have increased the age of jurisdiction, typically from 16 years old to 18, faced complaints from opponents worried that their juvenile system would be inundated with new cases and could collapse under the strain. But that hasn’t happened. Instead, the research shows that since 2007, the number of teens nationally who were automatically excluded from adult courts has been cut in half, from 175,000 to less than 90,000 today.

Researchers pointed to several reasons, including that juvenile crime continues to fall and that tens of millions of dollars originally earmarked for youth detention have been diverted to treatment and education, further helping to reduce crime.

Besides being less expensive than the costs of confinement, keeping teenagers out of adult facilities is safer for the juveniles, who often become victimized behind bars, the study said.

“I think the fact that you saw the number of states where 16- or 17-year-old kids are automatically excluded from juvenile courts cut in half is the most significant indicator of the speed in which things are changing,” said Jason Ziedenberg, director of research and policy at the institute and one of the study’s authors. “At this point, with all of the research, I think the reason these changes are being adopted is because it’s generally accepted as a truism that raising the age results in better outcomes.”

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Last year Louisiana and South Carolina changed their laws to allow offenders as old as 17 to remain in juvenile courts. Even before the change, Louisiana was aggressively working to increase its diversion and counseling programs at the local and parish level, the study found.

The study also took note of results in Massachusetts, Illinois and Connecticut. All three states predicted that raising the age of jurisdiction would result in much higher costs to taxpayers than actually occurred.

Connecticut has been among the most aggressive states in expanding the juvenile system, raising the age to 16 in 2010, and in 2012 made all teens up to 18 automatically eligible for the juvenile system. The state has seen a reduction in crime, recidivism for youth on probation has dropped nearly 20 percent and the state is closing its only youth correctional facility.

“What we’ve seen is so much of what people were worried about never actually happened,” said William H. Carbone, executive director of the Tow Youth Justice Institute at the University of New Haven. “We saw substantial reductions in juvenile crime since this went into effect. Over the years in our state, as we have reduced the number of kids in out-of-home placement, money has been invested into very effective, community-based programs such as cognitive behavior therapies and access to substance abuse and mental health treatment.

“By all accounts it’s been good for the individuals and good for the families involved,” he said. “It’s contributed to better economics, and we have closed four adult facilities in the last eight years, and by July of 2018 we will close the only juvenile detention facility in the state.”

Connecticut has been so pleased with the results, Carbone said, that the governor and legislature are working on a bill that will automatically place all offenders up to 21 years old into the juvenile justice system. Juvenile judges will retain the option of transferring the most severe cases to state court.

Last year, out of approximately 12,000 offenders entering the juvenile justice system, only 150 were transferred to adult courts, Carbone said.

“Along with declining crime, one of the reasons the projected costs of raising the age never materialized in Connecticut, Illinois, Massachusetts, or New Hampshire was that some cost estimates did not analyze the savings accrued from young people reoffending less often when served by the juvenile justice system versus the criminal justice system,” the study found. “Connecticut, Illinois, and Massachusetts outperformed the nationwide trend towards reducing the use of juvenile confinement, and outperformed the rest of the country in declines for violent juvenile crime and property crime arrests.”

Ziedenberg said the trend toward raising the age of jurisdiction will continue as more information about its benefits is made public. He said advocates now are focusing on New York and North Carolina, which currently exclude teenagers over 15 years old from the juvenile system. Those states, as well as Texas, are currently working on legislation to raise the age of jurisdiction.

“The momentum is there, and we want to continue this push,” he said. “I just think that is something that as we are able to share these facts — the safety for juveniles kept out of the adult system, costs and everything else — I think that is going to give advocates and legislators more information that will help them get the changes everyone knows are needed.”

This story was written for the Juvenile Justice Information Exchange, a national news site that covers the issue daily.