The Not-So-Certain Science of Pre-K

Frederick M. Hess
4 min readJun 23, 2022


There’s a lot to like about preschool. Over the years, my kids have used several different preschools — from a cheerful church basement to a hard-core Montessori — and had a number of really positive experiences. You don’t need to convince me that good, affordable, reliable, and convenient preschool is something we should value.

But it’s a big leap from there to concluding that preschool “works” or that universal pre-K is a good idea. Now, when I offer such qualifications, preschool aficionados immediately leap to patiently (or not-so-patiently) explain that I’m ignoring the science.

As for that science. Earlier this year, researchers at Vanderbilt University released the outcomes of a new gold-standard study of Tennessee’s statewide pre-K program, launched back in 2009. The study has been touted as the only randomized “thorough, ongoing investigation into the impacts of a statewide pre-K program for economically disadvantaged children.” The findings of the initial 2015 report were regarded as highly disappointing, with the benefits of the pre-K program fading before 1st grade. The follow-up results aren’t any better. The pre-K students fared worse on state assessments than their peers and had worse outcomes “for disciplinary infractions, attendance, and receipt of special education services.”

Readers who regularly hear that pre-K “works” might be surprised. They shouldn’t be. The truth is, an analysis of the 10 best-known, widely cited pre-K programs makes clear that the research around these programs shows not “that ‘pre-K works’” but that “some early childhood programs yield particular outcomes, sometimes, for some children.” Indeed, the most credible research has made clear that “the most meaningful, far-reaching effects” are the product of “carefully designed, well-implemented programs” — not sweeping mandates.

And even the studies that do indicate pre-K efforts were beneficial don’t necessarily point at a clear path forward for policies on pre-K. Consider the famed Perry Preschool project, a small pilot preschool and home-visiting program for 3- and 4-year-olds that ran from 1962 to 1967 in Ypsilanti, Michigan. The program provided 58 low-income Black children with two years of a research-based preschool curriculum and weekly home visits. It was found to have positive impacts on participants’ educational and life outcomes, reducing teenage pregnancy, and increasing lifetime earnings. All good stuff. But we’re talking about less than 60 students, in an intensive boutique pilot program, more than a half-century ago. The notion that Perry offers a clear blueprint for broad public policies is just fantastic.

And yet, some years back, Nobel Prize-winning economist James Heckman savaged a newspaper columnist for even questioning the benefits of expanded pre-K. Heckman insisted that it clearly works. How did he know this? Heckman pointed to his “evidence-based analysis of more than 30 years of data” from the Perry Preschool project. “It is as good a trial for effectiveness as those we currently rely on to evaluate prescription and over-the-counter drugs,” he wrote.

While Heckman is a very smart guy, he was wrong here. There’s a difference between a medical therapeutic and a public-policy response. Unlike a preschool program, the ingredients of an over-the-counter drug can be replicated exactly and administered to new patients with precise directions. In the case of Perry, it’s not especially clear just what it means to replicate the program. Was the intervention simply attending a preschool? Doubtful. Was it attending a “high quality” preschool? OK, but just what made Perry “high quality”? Was it the student-teacher ratio? Home visits? The curriculum? All of the above? How exactly does one know?

Unless we know, it’s tough to be confident that policies will deliver the desired results. Worse, the bigger the policy, the bigger the risk that getting things wrong will have adverse consequences. And yet much of the support for universal preschool proceeds with a blind assurance that leaves all such difficult questions aside. For instance, last fall, the Biden administration’s Build Back Better proposal for universal preschool contained sweeping new federal requirements that would’ve governed preschool education standards, credentials, and pay. There was little evidence behind any of this and actually more cause to fear the ways in which it might bureaucratize early-childhood education, squelch small church-based and neighborhood providers, drive up costs, and aggravate staffing challenges. We would’ve been far better served not by assertions of “the science says” but by serious discussions of uncertainty, trade-offs, and sensible compromise.

Helping ensure that families have access to reliable, trusted child-care options is a good idea. But the claims made about preschool — and especially about universal pre-K — tend to be overstated, the benefits are far from certain, and fail to consider that massive new programs can cause real harm. We should keep working to expand access to reliable, convenient, trusted preschool. But we should do so guided by good judgment, not by overreliance on not-so-certain science.

This post originally appeared on Rick Hess Straight Up.



Frederick M. Hess

Direct Ed Policy Studies at AEI. Teach a bit at Rice, UPenn, Harvard. Author of books like Cage-Busting Leadership and Spinning Wheels. Pen Ed Week's RHSU blog.