BATON ROUGE -- Students who are held back in eighth grade are more likely to commit a violent crime by the time they turn 25, according to a study of Louisiana students by the National Bureau of Economic Research.

The study, released late last year, examined nearly 23,000 students in Louisiana who were held back in eighth grade between 1998 and 2001.

The authors chose students who scored just one point below the cutoff score needed on English and math LEAP, or Louisiana Educational Assessment Program, tests to be promoted to ninth grade.

They compared those students' outcomes years later with students who scored just slightly higher and were promoted, in an attempt to control for socioeconomic factors that could also have contributed to higher criminal-conviction rates in later years.

The results were extreme: Students who were held back in eighth grade because they just barely missed a passing grade on the standardized tests were over 58 percent more likely to commit a violent crime by their mid-20s.

The policy of eighth-grade retention is costly, too, the study found. The increase in violent crimes, driven by grade retention, implies a social cost of between $2.6 million and $18.4 million, it said.

"The size of the crime effect is such that any (educational) benefits would have to be large to overcome the costs from more crime," wrote the authors, Ozkan Eren, Michael F. Lovenheim and Naci H. Mocan.

The study claims to be the first of its kind to link grade-based retention to violent crime later in life.

The authors found little effect on juvenile crime rates, which suggests the effects on adult criminal behavior are driven by fewer job market prospects and non-cognitive skills that stem from lower education levels.

Ken Bradford, an assistant superintendent in the Louisiana Department of Education, said he wasn't surprised by the results of the study.

In 2014, he and others created an optional transitional grade in Louisiana known as "T9," which allows some eighth-grade students who are not on track to pass the LEAP tests to continue their education on a high school campus, rather than being retained in middle school.

"What stood out to the department is 40 percent of retained eighth-graders were not arriving on a high school campus," Bradford said, meaning they were dropping out rather than repeat eighth grade. "They're youth that don't have education, going into society at this age."

Morris Jeff High School, which opened as an extension of the charter school's elementary and middle school, is among the schools in New Orleans that offer T9. Students there are able to graduate in four years if they stay on track with the program.

Patricia Perkins, the school's principal, said she believes in keeping students with their peers and providing in-grade interventions if they need help.

"It's that social-emotional piece and the stigma attached with keeping students back," Perkins said.

But in many places, the eighth-grade retention system is still used. First adopted in 1998, it requires eighth-grade students to achieve a certain level of proficiency on both the math and English LEAP exams to be promoted to ninth grade.

Students who score below the cutoff on either test are offered remedial summer classes and are required to retake the test in the summer. If they fail the summer exam, they are held back in eighth grade.

According to the authors, Louisiana was studied in part because it was an early adopter of the retention system -- although they note that the system has "grown markedly in popularity in the past several decades" to coincide with "the rise of accountability policies more broadly."

Sixteen states as well as many large school districts now have chosen to end automatic "social promotion." But, the authors say, the unintended consequences of such a move can be severe.

Students who fail to be promoted tend to attend high schools with lower-performing and more disadvantaged students, and retention in eighth grade increases truancy and disciplinary problems and causes students to be more likely to drop out of high school, the study says.

In Louisiana, the transitional program, T9, that could help prevent such problems still isn't used by most schools. Only 236 of the state's 1,422 schools have students in that transitional grade this year.

But Bradford said T9 is gaining traction, and he's hopeful that it will make a difference for the thousands of students already participating in it.

"It will be interesting to see: What type of impact did this have?" he said.