Fans of Catholic education were no doubt cheered to hear President Donald J. Trump’s shout out for school choice last night. Though details were lacking, Mr. Trump told Congress during his address on Feb. 28, “I am calling upon members of both parties to pass an education bill that funds school choice for disadvantaged youth, including millions of African American and Latino children. These families should be free to choose the public, private, charter, magnet, religious or home school that is right for them.”

Beyond the president’s endorsement, many other signs—the pro-voucher pedigree of newly minted Secretary of Education Betsy DeVos not least among them—indicate a vast new national arena for school voucher programs may be about to open. Voucher experiments have previously been restricted to children in low-scoring, high poverty districts, but what the president appears to prefer is a “let the money follow the student” approach for all U.S. schoolchildren that could revolutionize—or ruin, depending on your perspective—public education in America.

“I’d like to tweet, ‘Vouchers are killer; they’re the best’ or ‘Vouchers are sad!’” That may be easy to do in the “tweet world,” but in the real world, vouchers offer a complicated landscape to consider.

Throwing something of a damper on the voucher enthusiasm is a string of recent studies that have cast doubt on the impact at the individual level of choice experiments. Students who have left public schools, voucher in hand, for charter, private and religious schools have fared slightly worse than peers, according to these studies.

One report looking at outcomes in Ohio was published last summer by the Thomas B. Fordham Institute, generally a reliably pro-voucher public policy think tank. Students who use vouchers to attend private schools “fared worse academically compared to their closely matched peers attending public schools,” Fordham Institute researchers found, especially in math. “Such impacts also appear to persist over time,” the study reports, “suggesting that the results are not driven simply by the setbacks that typically accompany any change of school.”

Many caveats apply to the significance of this report and others like it, voucher supporters are quick to argue.

Daniel Hungerman is an associate professor of economics at the University of Notre Dame who has been looking at the impact of voucher programs on Catholic schools and parishes. “Pretty underwhelming” was his assessment of recent student outcomes at voucher-backed schools.

He told America, however, that other studies suggest that one collateral expectation of a switch to vouchers is indeed proving true in the real world. While students at voucher-supported schools may be faring slightly worse or no better than the kids who stayed behind in the public system, outcomes at those public schools appear to be ticking up, perhaps in response to the voucher-generated competition as school children “vote with their feet.”

Prof. Hungerman’s research has uncovered a different area of concern related to school choice programs. “On the one hand, it looks like vouchers can improve a church’s finances and help keep a school and a parish open, but we do see a decline in religious activity,” he said, including drooping donation levels from registered members as voucher windfalls begin to dominate parish programming and planning. Voucher dependency can be a worry.

He hastened to add, however, that his report, like the others, does not lend itself easily to a “simple tweetable conclusion.”

“I’d like to tweet, ‘Vouchers are killer; they’re the best’ or ‘Vouchers are sad!’” That may be easy to do in the “tweet world,” Hungerman said, but in the real world, vouchers offer a complicated landscape to consider.

“Vouchers might be able to help some struggling schools; they might be able to help some struggling families and students; they might be able to help some struggling churches.” But many programs, he said, come with significant strings attached, and voucher efforts can have unintended consequences, including “in the pews.”

“That doesn’t mean that churches should never try vouchers,” he said, “but they should be aware of all the possible effects when they do.”

For now, what Prof. Hungerman is willing to say with certainty is that more study is required before anyone can feel confident firing off a “sad” or “great” tweet about the impact of vouchers.

Kevin Clarke is America’s chief correspondent and the author of Oscar Romero: Love Must Win Out (Liturgical Press).