Everyone knows that children learn better in smaller classes — right? It’s a piece of educational wisdom touted by everyone from the president of New York’s United Federation of Teachers, Randi Weingarten, to the president of America. However, literally hundreds of studies have failed to show any convincing connection between reducing class sizes and improving students’ scores on standardized tests. This dearth of confirming data is something to keep in mind as Ms. Weingarten, a stakeholder in the Tweed Trust, pushes the idea of hiring more teachers — otherwise known as “dues-paying members” — to bring down the student-teacher ratio in the city.

America has been reducing class sizes steadily for the last half century, with little to show in results. A sketch of the key points is in a paper issued in 1998 by Eric Hanushek. Between 1950 and 1995, student-teacher ratios around the country fell by 35%. Solid national data are not available for all of those years, but student achievement, as measured by the National Assessment of Educational Progress, has remained generally stagnant from 1970 to today. With billions of dollars having been spent on class-size reduction at the federal and state levels over the past few decades, there seems to be little reason to anticipate any benefit from New York City upping the ante — especially in the middle of a budget crisis.

Proponents of small class size often point to an experiment undertaken in 1985 by the state of Tennessee. It’s the centerpiece of the Web page of the American Federation of Teachers — Ms. Weingarten’s parent union — on the class size issue. Unfortunately for these boosters, it doesn’t show any benefit to reducing class sizes past kindergarten. In the experiment, known as Project STAR, kindergarten students were randomly assigned to small classes, of 13 to 17 students, or large classes, of 21 to 25 students, and kept in small or large classes through third grade. The students in small classes pushed ahead slightly in kindergarten, but after that did not gain compared to their large-class counterparts — if there truly were a benefit in the later grades, the gap ought to have widened every year. Even more problematic, the experiment hardly isolated class size from other factors that could have been affecting student achievement; teachers and administrators involved in the program knew that it was being evaluated publicly and that future money for class-size reduction rested on its outcome, possibly leading to a placebo effect.

Far more scientific was a study done by noted educational economist Caroline Hoxby, an associate professor of economics at Harvard. Ms. Hoxby conducted a “natural experiment” involving 1,035 schools in 165 districts over 20 years in Connecticut. Using the natural fluctuation in the number of students entering a class each year, she created a sample with little statistical noise. A family might have one daughter with a first grade class of 25 students, and her younger brother, two years later, might have only 18 students in his first grade class. The students in this study were coming from the same economic background and sometimes had the same teachers. The result: no difference between students in small and large classes.

Even if one were to grant the most optimistic claims about small class sizes — small gains in the early grades — it is hard to see how this could justify a policy of spending scarce education dollars on a strategy that yields scarce results. Ms. Hoxby, in testimony before the House’s committee on education, said that reducing class size has a cost proportional to the reduction sought — meaning that a 10% reduction in class size requires about a 10% increase in spending for every student. Smaller classes require more teachers, more school buildings, and more administrators. This is quite an investment to make in an idea the results of which are so scant that they cannot be proven to exist.

Further, increasing the number of teachers isn’t guaranteed to reduce class sizes. The New York Post yesterday noted that while there are only 14 students for every teacher in New York City, class size averages 25 students; this is because of the various sabbaticals, professional days, and administrative tasks that suck teachers out of the classroom. Furthermore, an increase in the number of teaching positions citywide could result in the best teachers being poached by well-to-do schools from needy schools, a phenomenon seen in California.

Better, it would seem, to at least experiment with the concept of vouchers. While the mayor and the chancellor may be hesitant to go down this road, perhaps they will be less so if the Supreme Court gets around to removing one of the roadblocks that lies that way. The High Court’s decision to grant a writ of certiorari to a challenge to Washington State’s Blaine Amendment is a wake-up call for New York in that regard. It is one of the darker chapters in this state’s history that it gave in to anti-Catholic agitation by erecting a higher barrier of separation between church and state than that contained in the federal Constitution. And policymakers still raise Blaine objections to choice because of the danger that people might want to choose Catholic schools and Yeshivot. If the Supreme Court rules again for choice, there will be a much better, much more cost-effective road ahead than spending to reduce class size.