There's been surging national interest in private-school-voucher programs with the Trump administration's embrace of the idea.

But newer research on large-scale voucher programs has complicated the debate over private-school choice—policies which allow families to use public money or aid to attend private schools, including religious ones.

What does the research say? In a nutshell: The most recent findings are mixed, but they lean more toward negative.

I spoke at length with researchers from most of these studies for story I did on how private schools receiving public money in Florida face little state oversight.

Ed Week Investigates: 'There Is no Oversight' Private School Vouchers Can Leave Parents on Their Own

Studies out of Indiana, Louisiana, Ohio, and the District of Columbia have found that students, most of whom are low-income, fare worse academically after leaving their public schools.

But a separate study that looked at low-income students attending private school in Florida with state aid, found that students enrolled in college at higher rates than their peers in public school.

"I think the best evidence from the best recent research ... if anything, it looks like that maybe kids going to private school on voucher programs might do worse in reading and math than they do in public [schools]," said David Figlio, an economist at Northwestern University, whose study of vouchers in Ohio for low-income students attending poor-performing districts found voucher students performed significantly worse on state tests than their peers who were eligible for vouchers but remained in public schools.

His research on Florida's biggest private-school choice program—the Florida Tax Credit Scholarship—found that on average, attending a private school on an FTC scholarship had zero effect on student academic achievement—which was generally true of most early voucher research, said Figlio.

"There are possible explanations: they're getting a worse education ... they're getting a different form of education ... and I don't think we really know the truth," Figlio said. "But I think there's precious little evidence so far that these kids do better academically."

Similarly, negative results were found in a recent study of Washington D.C.'s voucher program as well—the only federally funded voucher program in the nation.

Students, at least in the Indiana and Louisiana voucher programs, recouped their academic losses after being in private schools for a few years.

Oversight and Demand

Indiana and Louisiana have something else in common that makes these findings even more interesting: They have some of the strictest oversight rules for private schools receiving voucher money in the country.

So at least in those two cases, increased oversight doesn't necessarily guarantee students will do better academically, nor do low test scores dampen parental demand.

In Indiana's voucher program for low- and middle-income students, the state assigns private schools letter grades—just like it does for public schools—and boots low-performers out of the program. All private schools in the state are required to be accredited.

But researchers from Notre Dame and the University of Kentucky examined data for Indiana voucher students in grades 5 through 8 and found that their math scores dropped significantly in the first few years of attending a private school. By the fourth year, students had recouped their losses, but about 25 percent of students in the researchers' sample had returned to public school in that period.

"We know that on average, those kids that switch back are lower-achieving and their achievement drops even more when they return to a public school, and that's not good," said Mark Berends, a sociology professor at Notre Dame and the director of the Center for Research on Educational Opportunities. But he is hesitant to draw conclusions. "This is still an analysis we are still working out ... It's intriguing, but we have more work to do."

In Louisiana's program for low-income students attending low-performing public schools, private schools that accept voucher students aren't allowed to pick and choose who they admit. Students must also take the state's standardized tests.

Similar to Indiana, students in Louisiana's voucher program performed much worse in both reading and math than their low-income peers who remained in low-performing public schools, according to two separate studies.

But low-test scores don't appear to deter parents from enrolling their students in the programs.

"[D]espite having very negative impacts on student performance, it's over supplied," said Christopher Walters, an economist at the University of California at Berkeley who studied Louisiana's vouchers with researchers from Duke University and MIT. "The parents could be interested in other school attributes [such as] religious instruction ... But one takeaway is that we shouldn't expect parents to make choices that improve student academic achievement."

Nor has it deterred lawmakers from expanding such programs.

Too Much Regulation?

Regulations could be part of the problem: they may actually drive down student academic performance in voucher programs by discouraging high-quality private schools from participating. At least that's one theory for what's happening in Louisiana, where state schools chief John White believes that private schools accepting vouchers be held to the same accountability standards as regular public and charter schools.

Only low-quality schools that are struggling to attract students may see vouchers—whatever the cost in autonomy—as an attractive deal, said Patrick Wolf, a professor of education policy at the University of Arkansas.

"It really is a bit of a Catch-22. I think good arguments can be made on either side of the regulation question," said Wolf, who co-authored the other study on Louisiana vouchers.

"Certainly one way to attract more high quality private schools is to offer more money and the other is to limit the regulations. The question is, is there really a stomach for that in the public community?"

A Silver Lining?

There is a silver lining to this cloud of negative findings, and it's in the Sunshine State. There, a recent study of the Florida Tax-Credit Scholarship, which gives hefty tax incentives to businesses that donate money to fund scholarships for low-income students to attend private schools, found that those students were more likely to go to college after high school than their peers that remained in public schools.

The longer students were enrolled in private schools, the larger the effects.

However, there are a couple of caveats.

"The kids who stick around longer are probably the kids for whom the program is more successful," said Matthew Chingos, the director of the Urban Institute's Education Policy Program. "That could manifest itself as just ... parents are not pulling them out because they feel it's working, but it could also manifest itself through the school being more likely to encourage them to stick around."

Additionally, Chingos' study couldn't eliminate selection bias—the idea that the most motivated families are likely to opt into school choice programs. (That was not an issue in the Louisiana studies because vouchers are awarded to students using a random lottery.)

Finally, researchers couldn't track students who attended private colleges or colleges outside of Florida, so more students could be going on to pursue a post-secondary education than counted in the study.

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