Universal prekindergarten sounds like a good thing. Early education for all! Why not? Anything for the kids.

Universal pre-k already exists—or is close to existing—in a number of states, including Oklahoma, Florida, and, most recently, New York. And given the appeal of the idea, it’s no wonder "preschool for all" emerged as a key talking point this election season, a year or so after President Barack Obama proposed a $75 billion federal universal pre-k program that involves partnerships with states.

The promise of universal pre-k figured prominently in the 2014 campaigns of gubernatorial candidates in Pennsylvania and Maryland, among others. In Hawaii, an ultimately unsuccessful ballot initiative that would’ve amended the state’s constitution to allow the government to contract with private preschool providers—and eventually implement a universal pre-k system—was described, repeatedly, as a "yes brainer."

But is universal pre-k truly the panacea that politicians and advocates, including Obama, make it out to be? Not quite, researchers say—although it does, as they point out, make for an effective political tool.

In fact, Ron Haskins, a preschool expert who co-directs the Center on Children and Families at the left-leaning Brookings Institute, went as far as calling universal pre-k "a very bad idea." Based on common sense alone, every young child should, ideally, have access to quality early-learning experiences before kindergarten, regardless of whether that kid’s family can afford it. Preschool can cost as much as $1,000 a month; sometimes it costs even more. Daycare, for its part, costs more on average than college in most states. The high cost helps explain why more than half of the country’s 3- and 4-year-olds miss out on preschool.

And the United States falls far behind other developed countries when it comes to preschool participation, ranking 26th in the world for the percentage of 4-year-olds enrolled in early-education programs. Many countries—including Japan, the United Kingdom, Mexico, and France—enroll nearly 100 percent of their 4-year-olds in preschool.

But the reality is that most states, along with the federal government, lack the money to fund preschool for all children. Even models that save money by using private-sector services are typically too expensive. And when efforts focus on creating across-the-board pre-k systems, the children who may be taking the biggest hit are the ones who would most benefit from a quality prekindergarten experience: those who are poor, speak English as a second language, or are otherwise disadvantaged.

Most states do fund pre-k to some extent, as do local school districts. A little over half of the state-funded programs have income requirements. During the 2012-13 school year, roughly a quarter of the country’s 4-year-olds—28 percent—were enrolled in state-funded pre-k programs, according to data from the National Institute for Early Education Research, or NIEER. (This interactive New York Times graphic offers a good breakdown of how state-funded pre-k varies across the country, though the data is slightly outdated.)

State-funded pre-k is, of course, one of a patchwork of early education services available to families. There are the Head Start centers for low-income children, which are typically operated locally by private nonprofits and for-profits and funded largely by a mixture of federal and state dollars, and private preschools and daycare centers, among other offerings.

Still, the level of funding is hardly high enough to provide for all preschool-aged children. The funding shortfall, moreover, undermines the quality and effectiveness of existing programs. By expanding pre-k for all, the money could be spread even thinner.

Some experts doubt the value of universal pre-k altogether, including Neal McCluskey, associate director of the Center on Educational Freedom at the libertarian Cato Institute.

"The reality is there isn’t good research basis to say that pre-k is good," McCluskey said, pointing to faulty data and the limited scope of studies on the long-term benefits of early learning. "Preschool has been oversold. People too often speak as if it’s a certainty that preschool has strong, lasting benefits."

But other critics of universal pre-k say that’s not the point.

"You have to look at the trade-off," said Darleen Opfer, the education director at the RAND Corporation, a policy think tank. "If you have a state that can’t afford high-quality preschool for everyone, where does the investment really make sense? To me it’s not an issue of whether or not [pre-k is] a good thing," she continued. "The clashes come over how to do it."

Indeed, though research on the long-term benefits of preschool is limited and widely debated, it’s clear that having a quality early education can significantly enhance a child’s life, at least in the short term, and help close the ever-widening achievement gap.  

"It definitely pays off—particularly for low-income kids," Opfer said, citing research showing that children who enroll in high-quality pre-k programs can go on to earn incomes as much as 23 percent higher than what they’d earn had they not gone to preschool.

The reason early-learning programs have greater impact for disadvantaged kids is that they often provide stimulating environments that the children don’t get at home. At preschool, they’re more likely to develop broader vocabularies and other developmental language skills. And they’re more likely to have better nutrition, according to Opfer.

A middle-class kindergartner who comes in with prior schooling has a significant edge over a low-income kindergartner who doesn’t—and that disparity increases as the kids work their way up the pipeline, says Larry Cuban, a former high school teacher and professor emeritus of education at Stanford University.

Some small-scale studies indicate that a person with a preschool education is less likely to drop out of high school, get arrested, repeat grades, and require special education services, suggesting that government-sponsored preschool is a financial investment worth making. The return on investment for every $1 spent on preschool ranges from $4 to $17, according to Opfer. And it really comes down to the quality of the program and whether or not it’s targeted at the families most in need, she said.

Even Head Start—a federal program that provides preschool and social services to low-income families—has mixed results, with kids’ gains seeming to fade over time, particularly if their elementary education is low-quality. Moreover, the program’s budget has been repeatedly subject to cuts over the years, reducing the number of children it’s able to serve.

Steven Barnett, director of NIEER, criticized the 50-year-old Head Start program for falling short of its mission to serve poor children with quality education, pointing to low standards and the program’s inability to consistently target the neediest kids, which make up a constantly evolving demographic. Families often go in and out of poverty—their economic status is rarely static, Barnett said.

Meanwhile, many of the country’s existing private preschools are little more than glorified daycare centers. Their staffs often consist of unskilled, low-paid employees who work under the guise of classroom teachers. They don’t use prepared lesson plans; they don’t focus on developing the cognitive, physical, and social skills expected of today’s kindergartners; they don’t have the kinds of facilities a quality classroom needs.

So why the push for universal pre-k?

Bruce Fuller, a researcher at the University of California, Berkeley, says the movement traces back to shifts in political priorities in the ‘90s, when groups ranging from teachers unions to private foundations called for greater emphasis on middle-class children. But analysts have yet to prove that expanding middle-class access to preschool has as much impact as it does for low-income children, Fuller said. Fuller compared state-funded pre-k for middle-class kids to a "water-down penicillin shot" and emphasized that the onus of enhancing students’ educational experiences is also on K-12 schools.

"The problem is that the advocates from the ‘90s forward talk about pre-k as an inoculation," Fuller said. "As if somehow that’s going to inoculate them from downstream mediocrity."

But Barnett, who supports the universal model and believes pre-k should be integrated into the K-12 system, pointed in part to the value children gain from interacting with peers who come from different backgrounds. He also stressed that middle-class kids struggle, too.

"While the impacts of preschool may not be as large for middle-income kids, they’re still substantive," Barnett said. "We’d be more successful in dealing with the problems of school failure, poor health, crime if we had programs for everybody than we would if we serve just low-income kids."

Barnett and other experts also pointed to the political advantages of universal pre-k.

Haskins cited the theory that an all-inclusive system would be more  "politically protected" because wealthier families would have the money to help bolster any government-sponsored program. Opfer noted that the states where universal preschool has been implemented were able to do so largely because they got middle- and upper-income parents to vote for it.

Cuban agreed: "It’s politically impossible to get it passed if it’s only aimed at poor kids of color, so what you do is you spread it to everyone so that white, middle-class families have access. It becomes a political possibility when you make it available to every family.