As congressional Democrats gear up for a second shot at passing Build Back Better — or at least select parts of it — special attention should be paid to the president’s desire to aggressively subsidize pre-K and child care. And by aggressively, I mean that the president has claimed savings in excess of $10,000 a year for the typical family under his vision for the law.

During last year’s debate over the plan, critics largely focused on the practical aspects of such proposals, including their treatment of religious providers, the staggering costs to the government and the potential that they could increase costs for some families. 

But a new study published in Developmental Psychology raises a deeper issue: Pre-K programs can actually be detrimental to kids. 

The study is strong scientifically because it relies on a randomized experiment. 

Some Tennessee preschool sites were “oversubscribed,” and the program granted admission through a random draw. 

The researchers were able to track the kids involved over time, comparing those who’d been admitted with those who had not.

The study is also highly relevant to any plan to dramatically expand pre-K because Tennessee’s program is of a pretty high quality. Tennessee pays pre-K teachers at parity with elementary-school teachers, for example, and requires teachers to have bachelor’s degrees and certification.

And yet the program failed. 

It didn’t just fail to produce lasting gains; by sixth grade, kids admitted actually did worse. They scored lower on math, reading and science tests. 

They had worse attendance and discipline records. Simply put, by taking kids out of other care arrangements and placing them in a pre-K program, Tennessee’s efforts harmed these kids’ outcomes. Now the fact that this program failed doesn’t mean all pre-K programs will fail. 

The overall academic literature on this subject is mixed, and pre-K supporters can certainly find other examples to support their preferences, from the famous Perry Preschool and Abecedarian projects from decades ago to more recent positive results out of ­Boston. 

What the study does show, however, is that pre-K programs can hurt as easily as they can help — and we simply don’t know how to make a program that’s guaranteed to help. 

As the authors of the new study discuss, there’s little consensus as to exactly what aspects of a program are most important, and figuring that out will take a lot of further research. 

One possibility they raise is that pre-K programs would do better if they focused more on developing “unconstrained” skills, such as attention and working memory, as opposed to more “finite” skills like learning the alphabet. 

Another key part of the puzzle is that pre-K competes with whatever care a child would have received otherwise, and that care is not always bad. Few pre-K programs can improve on, for example, a capable family member who’s heavily invested in the child’s well-being. 

Expanding pre-K before we’ve figured all that out is an incredibly risky proposition. And a similar issue arises for child care for younger kids. 

Most notoriously, a big child-care expansion in Quebec appears to have had negative long-term effects on crime rates and health, according to research by Michael Baker, Jonathan Gruber and Kevin Milligan. 

Of course, pre-K and child care have effects on parents as well as kids, which surely loom large in the Democrats’ thinking. 

Any parent who has paid for child care wouldn’t mind paying less, which is part of the political appeal here. But by specifically funding child care and pre-K rather than supporting all parents equally, these subsidies privilege some types of family arrangements over others.

Stay-at-home parents get the short end of the stick. When a parent stays home, that parent eliminates the need for someone else to watch the kids. In a sense, these parents are “paid” by not having to pay for outside child care. When the government provides funding for parents who use child care but not parents who watch their own kids, it eliminates this benefit and tilts incentives in favor of both parents working — at the risk of pushing kids into care arrangements that, as the Tennessee study shows, might be worse for them. 

Warping incentives is no unintended consequence. Advocates of pre-K and child-care subsidies are sometimes explicit about this goal, framing the effect as “promoting gender equity” or “enabling women to work.”

Of course, it’s true that most stay-at-home parents are moms. But I spent about a year and a half as a stay-at-home father myself, so this is not an issue purely about gender.

At root, this is really a question of whether the government should intentionally, and massively, subsidize some child-care decisions over others, very possibly with detrimental effects for kids.

Robert VerBruggen is a fellow at the Manhattan Institute.