It’s no wonder Republican and Democratic voters, divided on nearly all policy initiatives, agree on the importance of pre-K. Nearly nine in 10 parents support the idea of optional public pre-K in the U.S., as do 8 in 10 of people without children, according to a recent survey. And about 70% of Americans agree that federal funds should be used to pay for these programs. 

President Joe Biden has made the topic a rallying cry of his presidency. He put forth a $110 billion proposal for free, high-quality pre-K in his now flat-lined Build Back Better package. “Universal pre-K for 3- and 4-year-olds is not just an investment in education, it’s an investment in the future of America,” he wrote on Twitter last year. 

The idea of free childcare is particularly appealing to parents who have struggled to keep up with the rising cost of childcare in the U.S., which increased by more than 40% during the pandemic. The problem is that real-world data is puncturing these big dreams.

A new, multi-year study published this month by researchers at Vanderbilt University’s Peabody Research Institute issued a biggest blow to universal pre-K. It found that students who attended state-funded pre-K programs in Tennessee experienced “significantly negative effects” compared to the students who did not. 

By third grade, the researchers began to see a surprising divergence between kids who had attended pre-K and those who hadn’t. By the end of sixth grade, the study found, students who attended voluntary pre-K programs were doing worse than their peers in academic achievement and discipline issues. 

The study followed 2,990 low-income children in Tennessee schools to evaluate the long-term impact of the state’s public school-attached pre-K programs against a group of similarly low-income children who did not attend the programs. The control and test group also evaluated, through a series of interviews with participants, whether other factors like parental involvement were equal, the researchers said. 

The negative outcomes for those Tennessee children might stem from public school pre-K programs taught using public school curriculum. Pre-K should not be kindergarten light, Dale Farran, director of Vanderbilt University’s Peabody Research Institute and one of the authors of the study, told Fortune. It should have an entirely unique curriculum that focuses on care and tending. If education officials put 20 small children in a classroom and tell them to stay quiet and still, it hurts them on a developmental level, according to Farran.

Still, Farran said she was shocked.

“We were all thinking that this study was going to be the first rigorous study that would validate pre-K,” she said. “And when the kids entered pre-K, the results looked good, they looked like other studies of pre-k. Kids were ahead on school readiness skills over those who didn’t attend, but by the end of kindergarten, those differences had gone away.” 

Causation versus correlation 

It took two years to get the most recent version of the study accepted, said Farran, because people had so much trouble accepting that students who attended pre-K could experience worse outcomes than students who had not. 

“They made us go back and do robustness checks every which way from Sunday,” said Farran. The new study includes 26 supplementary tables to test all possible explanations for the divergence. But even with all of these control experiments, the results remained the same. 

“If our results came out the other way, no one would ask us to explain them,” said Farran. “When a study doesn’t work, then people rightfully ask you to explain why.” 

Conclusions about the effectiveness of universal pre-k should not be drawn from one study, warned Beth Meloy, president of Meloy Child and Family Policy Solutions, which consults for the Early Learning and Care Division at the California Department of Education. 

“We need to look into the context of these studies,” she told Fortune. “There are many factors that influence a child’s development and later academic achievement.” Softer social skills are often unaccounted for as well, she said. It’s hard to measure skills that pre-k might impart onto students like executive function or critical thinking. 

The problem could also be Tennessee-specific. The quality of the program also really matters, said Meloy. If states don’t devote adequate funding to hire well-trained teachers, or design a curriculum that incorporates play and learning, the long-lasting impacts of pre-k could be skewed. 

A May 2021 study by researchers at MIT’s Department of Economics found that there were numerous long-lasting benefits for pre-k students in Boston. The short-term impacts of preschool are evident in student behavior but not test scores. But in the long term, the researchers found that students who attended pre-k in Boston were 6% more likely to graduate high school than those who didn’t, 8.5% more likely to take the SAT, and 8.3% more likely to enroll in college immediately following high school graduation. 

“As policymakers consider increased public investment in universal preschool, the research findings suggest that preschool can lead to long-term educational attainment gains through improvements in behavior,” the study concluded. Massachusetts’s public education system is ranked number two in the country. Tennessee, meanwhile ranks in the bottom five states for education funding and the bottom half for public education overall. 

There have been other studies that find the benefits of pre-K diminish quickly after children enter kindergarten, but those tend to focus exclusively on test scores or grades as a metric. 

“The reality is that learning programs that aren’t designed to meet children at their developmental level and provide the kind of activities to scaffold the learning children need at any given age do have the potential to not support a child’s growth and development,” said Meloy. 

A different 2014 study that Farran worked on found that there was a “great variation” in the quality scores of classrooms across Tennessee, and that 85% of classrooms scored below the level of “good” quality. In another New York Times’ op-ed, Farran said that pre-k classrooms in Tennessee lack a “coherent vision” and leave teachers to “their own devices” to figure out how to guide their students. 

Still, Farran told Fortune that she believes policymakers are relying too much on the concept of pre-k as a way to cure educational disparities that stem from wealth inequality in the U.S.  

Policymakers, she said, think they’ve “got this magic bullet: We’re going to cure poverty. we’re going to cure the achievement gap, we’re going to have more retention in school, and fewer Special Education referrals if we just add nine months of pre K at five and a half hours a day.”

Unfortunately, added Farran, magic bullets don’t exist.