A new study by an outspoken critic of academic grade inflation finds that 43 percent of all letter grades issued by professors today are As.

Stuart Rojstaczer has spent years amassing evidence of grade inflation, an on-again, off-again trend he traces to 1940.

By Rojstaczer’s reckoning, the A has emerged in the new millennium as the most popular grade — it eclipsed the B in prevalence around 1998. His study, published in the journal Teachers College Record, reveals a slow but steady transformation of grades and grading in academia.

Consider: In the World War II era, roughly 35 percent of grades were B, and an equal share were C. Just 15 percent were A. A slightly lower share were D, and 5 percent were F.

And today? Nearly half of grades are A. The bland B has held steady at 35 percent. The C has declined just as sharply as the A has ascended, and it now accounts for just 15 percent of all grades.

To put it another way: 50 and 60 years ago, B’s and C’s made up the vast majority of all grades. Today, the most common grades by far are A’s and B’s.

What is going on? Rojstaczer and co-author Christopher Healy suggest that the motive is a “consumer-based” approach to higher education: Particularly in private colleges, professors grade high to keep students satisfied with their collegiate experience.

They track grade inflation alongside the rise of faculty evaluations by students. Faculty who grade well get better reviews from their student “customers.” Those evaluations, in turn, determine faculty pay and promotion.

The authors contend that colleges are effectively losing their standards. There is no distinction any more between good and excellent performance; students at the average college will get a high grade no matter how much classwork they complete or how well they score on their tests.

Over the past few decades, they write, “America’s institutions of higher learning gradually created a fiction that excellence was common and that failure was virtually nonexistent.”

They theorize that the promise of an easy A might, in turn, prompt students to spend less time studying and working on class assignments, another well-documented trend over the past 50 years.

Absent firm oversight, they write, “meaningful grades will not return to the American academy.”