An examination of students enrolled in Tennessee's preschool program underscored earlier studies that suggest preschool's gains may not last.

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For decades, researchers studying preschool have struggled to explain a common finding: While kids who go to preschool arrive in kindergarten better prepared than those who don’t, the reading and math test scores for both groups look about the same by about the time they reach third grade.

Some call the phenomenon “fade-out,” saying that preschool’s benefits don’t last.

Others call it “convergence,” suggesting that elementary school teachers may be good at catching up kids who didn’t attend preschool — or they might be boring the kids who did, stifling their potential.

It’s not a universal finding — a 2014 study of Washington state’s preschool program for low-income children found that gains persisted through fifth grade.

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And Seattle’s new preschool program, which voters approved last November, hopes to do at least as well. It got under way this fall.

But the latest results from a major study, released earlier this week, underscored the notion that preschool gains can fade out fast — and offers some cautions about moving ahead too quickly without nailing down quality first.

The 5-year study from Vanderbilt University, which is following a random sample of low-income students in Tennessee who attended that state’s preschool program, has found the same flattening out of test scores, with a troubling twist.

Children who attended preschool started kindergarten ahead on many readiness measures, but after first grade, they consistently showed somewhat lower gains on test scores and social and emotional skills than the kids who didn’t attend.

“The negative trend showed up on all the measures,” said Dale Farran, one of the study’s co-authors. “We just didn’t expect to see that.”

Researchers can’t say what caused the downturn, but Farran said that surveys of teachers involved in the study suggest one possible answer: some kids get burned out because their preschool experience wasn’t high-quality, with teachers doing most of the talking in whole-group lessons that aren’t well suited for four-year-olds.

“It’s a little turning point, possibly, at the end of first grade where the teachers are giving us some hint about what’s going wrong,” Farran said. “That the children have been exposed to this sort of rigid, behavioral control … for now three years in a row.”

Farran said that Tennessee rolled out its preschool program quickly, and ended up with classrooms that varied greatly in instructional approach.

“If you just scale up quickly — which is what we’re doing all over the country — with minimum cost and minimum oversight, you have no idea what people are doing out there,” Farran said.

The quality of the Tennessee preschool classrooms was possibly lower than in much-studied city preschool programs in Boston and Tulsa, but on par with large state-run programs and national studies of the Head Start program, Farran said.

But no other study has found a decline in test scores for kids who attend preschool, said Steven Barnett, director of the National Institute for Early Education Research.

But he argued that the findings reflect a flaw in the study, comparing groups of kids who have important differences. The authors strongly disagree.

Seattle is emphasizing quality over quantity, ramping up slowly to serve 2,000 children in 100 classrooms by 2018,

And in Washington, former state Rep. Ross Hunter, who became the state’s Director of Early Learning this summer, said that improving preschool as well as K-3 education is key to sustaining early gains.

“The lesson I would take from (the Tennessee study) is to focus in on the quality in the early learning programs, as Washington is doing,” he said.