Perhaps preschool doesn't help children as much as we thought — or hoped.

A new study by Mark Lipsey, Dale Farran, and Kerry Hofer finds that children who were admitted to Tennessee's pre-K program were worse off by the end of first grade than children who didn't make the cut.

The study is beautifully designed — it takes advantage of areas in Tennessee where demand for the program outstripped supply, so entrance to the program was decided randomly. That means researchers could compare outcomes for kids who randomly got in with outcomes for those who randomly didn't, and isolate the effects of the program. What they found should worry advocates of universal pre-k.

Could bad pre-K actually hurt kids?

At the end of pre-K, the results look pretty much as you would expect: Teachers rates the children who went through pre-K as "being better prepared for kindergarten work, as having better behaviors related to learning in the classroom and as having more positive peer relations."

The problem is those results dissipate by the end of kindergarten — by then, the group that attended pre-K is no better off than the group that didn't — and then begin to reverse by the end of first grade. By the end of second grade, the children who attended the pre-K program are scoring lower on both behavioral and academic measures than the children who didn't.

The researchers admit they're "perplexed" by their findings, but note that their results echo the findings of the Head Start Impact Study, which was also an unusually well-designed, randomized experiment. And while the researchers don't bring it up, their findings also echo new evidence out of Quebec, which launched a massive day care program that was successful in signing children up, but seems to have slightly hurt them over time.

The big question: Will the negative effects last?

All this flies in the face of earlier research showing tremendous, lasting benefits for pre-K. But that research consists of evaluations of programs that existed 40 or 50 years ago. The enrollments were very small, and the programs themselves were very high-quality (and cost a lot upfront). Sadly, Tennessee's pre-K is a lot closer to what we're likely to get with universal pre-K than the Perry Preschool Project, the most famous of those small, high-quality programs.

That said, Nobel Prize–winning economist James Heckman — who has, in recent years, been one of the biggest boosters of universal pre-K — says this study doesn't change his mind about the value of these programs; instead, he argues that follow-up studies looking at children's behavior in third grade are likely missing longer-term benefits:

The Perry Preschool program did not show any positive IQ effects just a few years following the program. Upon decades of follow-ups, however, we continue to see extremely encouraging results along dimensions such as schooling, earnings, reduced involvement in crime and better health. The truly remarkable impacts of Perry were not seen until much later in the lives of participants. Similarly, the most recent Head Start Impact Study (HSIS) seemingly shows parity at third grade while numerous long-term, quasi-experimental studies find Head Start children to attend more years of schooling, earn higher incomes, live healthier, and engage less in criminal behavior. Considering this, it is especially important that we see HSIS through before condemning Head Start. The decision to judge programs based on third grade test scores dismisses the full range of skills and capacities developed through early childhood education that strongly contribute to future achievement and life outcomes. The success of an early childhood program ultimately comes down to what is being evaluated, and too many evaluate the wrong things. Too many measure only half the child, focusing on IQ and cognitive gains at the expense of social and emotional skills that are often stronger determinants of adult success. Conscientiousness, self-control, motivation, persistence and sociability have far greater influence on full-time employment, lifetime wages, health, family and social outcomes than IQ and cognitive skills.

I'm not an expert in this field, but if Heckman is right, it will take a long time to prove that he's right. For now, results like the ones we're seeing out of Tennessee and Quebec have certainly put a damper on my enthusiasm for universal pre-K.

Go deeper: