It is difficult to pinpoint an exact moment when grades in American higher education began to inflate, but the smart money is on the late 1960s.

Sometimes grade inflation was driven by ideology. A few disaffected professors in the 1960s expressed their disdain for society's values by giving every student in their classes the same grade, usually an A. The students rarely complained.

Sometimes grade inflation was driven by something more basic. Many college teachers at the time — especially untenured junior professors — felt vulnerable in the wake of the near collapse of the so-called "core curriculum."

Before 1960, the core curriculum was a set of required courses that were intended to embody, however imperfectly, a school's vision of what it thought any educated person ought to know. It seemed reasonable at the time to expect that all educated people could tell a gene from a chromosome and had read some of the great classics of philosophy and literature.

As a result, a large number of professors taught courses that were required. A never-ending supply of students for such courses seemed guaranteed, even if the professors who taught them were notoriously demanding graders.

But in the wake of the social upheaval of the 1960s, the fragile consensus on what constitutes a liberally educated human being dissolved. Formerly required courses were now made elective — sometimes even marginalized — and the core curriculum became a pale shadow of its former self.

Which meant that professors who taught difficult subjects or were known as tough graders found themselves with a sharply reduced student enrollment and in very real danger of being thought expendable by an administration strapped for cash.

Part of the strategy for luring students back into these thinly populated courses was to flatter them by lowering the standards for higher grades. The very best students still received A's (there was, after all, no higher grade to give them), but good students received A's as well instead of the B's they had earned. Average students were rewarded with B's for work that was at best satisfactory, while C became a negative grade for work that was passing but vaguely unsatisfying.

Grade inflation did not spare the Ivy League. Last year the University of Pennsylvania gave A's to 54 percent of its students, while Harvard gave A's to 48.7 percent, Brown to 46.7 percent, and Princeton to 40.9 percent. The lowest percentage was Yale's, which hovers around 30 percent, the number it allows to graduate with honors.

Some professors at elite schools argue that grade inflation doesn't matter or, at any rate, doesn't matter very much. Elite schools attract many of the best students in America, a fact that skews to the high side the number of A's given by such schools in any semester.

To some extent, their argument is hard to dispute. Great courses, from classics to cell biology, are still being taught in American colleges and universities, and bright students who deserve A's are still receiving them — even if A's are being awarded to far too many students who do not.

What does matter, however, is truth-telling and the credibility of a school's grading process. Higher education likes to think of itself as a truth-telling business and insists that tenure is important to protect the tellers of unpopular truths. Harvard and Yale even thought that truth-telling was so important that they put the Latin word for "truth" into their school mottoes. But in their grading practices, American universities have a checkered reputation for candor.

Calling "average" work "good" and "good" work "excellent" is a lie, even if a well-meaning one. By telling this lie, universities are not altering reality, but only postponing the day of reckoning for students misled by a too-generous assessment of their work.

At the same time, schools that tolerate inflated grades are depriving genuinely excellent students of the recognition they so richly deserve. "After all," a bright student might reasonably ask, "what does my hard-won A mean if everyone in the class got one?"

To tell the truth, not very much.

David C. Steinmetz is the Amos Ragan Kearns Professor of the History of Christianity at the Divinity School of Duke University in Durham, N.C. He wrote this commentary for the Orlando (Fla.)Sentinel.