Recent research has shown that teachers are by far the most important factor in student achievement—some 20 times as important as other factors, like class size or socioeconomic status.

Varying significantly in their capacity to teach, some teachers consistently produce large gains in student achievement, regardless of the kinds of students assigned to them, while other teachers do not.

Thus it is especially troubling that, in recent decades, those entering the teaching field have, on average, lower measured aptitudes than did their predecessors. This decline in the average ability of our teachers—it is reasonable to conclude—is a major factor in the decline of American public schools.

That teaching, by and large, no longer attracts exceptional applicants, is clear. A recent study by three University of Maryland economists found that the likelihood of highly talented females becoming teachers fell from roughly 20 percent in 1964 to just over 11 percent in 2000. In New Jersey, the commissioner of education surveyed his state’s teacher colleges and found that, for prospective teachers, the verbal and math SAT scores, when combined, were lower than 800 out of a possible score of 1,600. Finally, the Educational Testing Service found that those indicating they intended to become teachers score near the bottom among those taking the ETS tests.

So what accounts for the increasing flight of talented women from teaching? Conventional wisdom has long suggested that the primary explanation must lie with the new opportunities available elsewhere for college-educated women. Since other factors also have an effect, however, what has been needed is an approach for measuring the factors’ relative power.

Now, such an approach to the question has been produced by Harvard economist Caroline Hoxby and Australian economist Andrew Leigh. And their important new study strongly suggests that, on this question, the conventional wisdom has been quite wrong.

Economists call it pay “compression” when salary schedules require those with the highest aptitude to earn no more than those with the lowest. Since the 1960s the rise of collective bargaining within education has brought increasing compression to the pay scales of public school teachers.

Moreover, teachers under collective bargaining contracts—here in Nevada and around most of the U.S.—are not compensated according to how well they teach. Instead, school districts are required under their union contracts to compensate teachers according to years on the job and number of (often irrelevant) academic credits. In consequence, teaching in America is the one profession where pay has been essentially decoupled from performance.

And it is this decoupling, say Hoxby and Leigh, which has been the biggest factor, by far, in the flight of high-aptitude women from the field of public-school teaching.

“We were inclined to accept the conventional wisdom when we began this project,” wrote the two scholars in a recent article in Education Next, the peer-reviewed education-reform journal of Stanford’s Hoover Institution.

However, say the economists, “we found, surprisingly enough, that pay compression within the teaching profession, induced by the introduction of collective bargaining, has had by far the greater effect.”

As a proxy for an individual teacher’s aptitude, the study used average aptitude of all students at the college from which she graduated. Highly selective colleges, for example, were defined as those where average student SAT scores were in the top 5 percentiles. Bottom-tier colleges, on the other hand, were those where average student SAT scores were in the lowest 25 percentiles.

Between 1963 and 2000, the share of new female teachers from bottom-tier colleges more than doubled, rising from 16 to 36 percent. Over the same period, the share of new female teachers who came from highly selective colleges fell from 5 percent to 1 percent—a five-fold drop.

During the same period, earnings of teachers in the lowest aptitude group rose dramatically relative to the average, so that teachers who in 1963 earned 73 percent of the average salary for teachers could expect to earn exactly the average by 2000. Meanwhile, earnings of high-aptitude teachers fell dramatically, as a ratio to average teacher earnings. In states where in 1963 graduates of the most highly selective colleges were making 157 percent of the average salary, they were, by 2000, earning no more as teachers than did graduates from the least selective schools.

The results, here in Nevada and all across America, have become too clear.

Union-imposed pay compression drove many of the best teachers out of teaching—turning public schools increasingly into a sanctuary for mediocrity and a conspiracy against the outstanding.

Steven Miller is policy director for the Nevada Policy Research Institute.