New York City Mayor Michael Bloomberg said last week that if it were up to him, he’d double class size and fire the 50 percent of teachers who are in the bottom half of effectiveness ratings: “doubl[ing] the class size with a better teacher is a good deal for the students.” Bloomberg, in his inimitable way, breezily insulted 80,000 teachers to make a point unsubstantiated by any social-science evidence.

When I heard about the mayor’s remarks to students at MIT, I was reminded of a school finance court case in Maryland some 30 years ago for which I served as a consultant. (At the time, I was a young doctoral student.) The poorest districts in the state, including Baltimore City, were suing the state to force it to equalize school funding. The state was joined by Montgomery County, the wealthiest district in the state, and hired noted economist Eric Hanushek to testify that money doesn’t make a difference in student outcomes. I was hired to prepare questions for cross-examination that might discredit Hanushek’s testimony.

A friend—then a sociologist and now a renowned specialty seafood purveyor—suggested a novel line: If, as Hanushek argued, spending more money wouldn’t increase achievement, wouldn’t spending less money have no effect on achievement either? “Brilliant!” I thought.

The time came for the cross-examination, and, among many other questions, the plaintiffs’ attorney asked this question.

“That—that almost follows,” Hanushek replied.

He went on to argue that increasing and lowering spending probably weren’t symmetric, and cutting spending would pose many political and practical obstacles; there was more evidence about what would happen if spending were increased, because spending had been steadily increasing in some districts.

It wasn’t the Perry Mason moment I was hoping for, but it was enlightening nevertheless. The reality is that Hanushek’s claim that money doesn’t matter was based on natural variations among districts in their spending patterns, and wealthy districts spent more and had better educational outcomes than poorer districts that spent less. There were no studies showing what would happen if spending were to increase or decrease precipitously over time (due to court intervention or something else). Ultimately, the judge’s decision hinged more on the language in the Maryland state constitution than the social-science evidence introduced during the litigation.

Class-size reduction is a complicated technical, political and moral issue, and I’ve written previously about it here, here, here and here. But almost all of the attention has been on the consequences of lowering class size; to my knowledge, there’s never been a study that has sought to understand the consequences of experimentally raising class size. And there is little reason to think that class-size reduction and class-size augmentation are symmetric in their consequences. Consider, for example, a teacher with a class of 30 students who assigns and grades homework nightly. Is the increase in the quality and quantity of teacher comments for a class of 15 students proportional to the decrease in the quality and quantity of comments when there are 60 students in the class? Are there class-size thresholds above which classroom management becomes impossible?

Mayor Bloomberg’s comments are not surprising—a little bit of knowledge is a dangerous thing, and he has a passing familiarity with the social-science evidence on education and social-policy issues. My only puzzlement is that he was so timid. Fire half the teachers and double class size? Why stop there? If teacher quality is the single most important thing about schooling, and class size is irrelevant, why not fire the bottom three-quarters of teachers and quadruple class size? Why wouldn’t that be an even better deal for students? I thought Bloomberg was supposed to be a bold thinker. Guess I was wrong.

Eric Hanushek argued that the evidence indicated that throwing money at schools wouldn’t influence student achievement. Would, as Bloomberg contends, the same be true for throwing students at classrooms? That—that almost follows.