We’ve written before about some of the work of Stuart Rojstaczer and Christopher Healy, grade inflation chroniclers extraordinaire. They have put together a new, comprehensive study of college grading over the decades, and let me tell you, it is a doozy.

The researchers collected historical data on letter grades awarded by more than 200 four-year colleges and universities. Their analysis (published in the Teachers College Record) confirms that the share of A grades awarded has skyrocketed over the years. Take a look at the red line in the chart below, which refers to the share of grades given that are A’s:

Stuart Rojstaczer and Christopher Healy

Most recently, about 43 percent of all letter grades given were A’s, an increase of 28 percentage points since 1960 and 12 percentage points since 1988. The distribution of B’s has stayed relatively constant; the growing share of A’s instead comes at the expense of a shrinking share of C’s, D’s and F’s. In fact, only about 10 percent of grades awarded are D’s and F’s.

As we have written before, private colleges and universities are by far the biggest offenders on grade inflation, even when you compare private schools to equally selective public schools. Here’s another chart showing the grading curves for public versus private schools in the years 1960, 1980 and 2007:

Stuart Rojstaczer and Christopher Healy

As you can see, public and private school grading curves started out as relatively similar, and gradually pulled further apart. Both types of institutions made their curves easier over time, but private schools made their grades much easier.

By the end of the last decade, A’s and B’s represented 73 percent of all grades awarded at public schools, and 86 percent of all grades awarded at private schools, according to the database compiled by Mr. Rojstaczer and Mr. Healy. (Mr. Rojstaczer is a former Duke geophysics professor, and Mr. Healy is a computer science professor at Furman University.)

Southern schools have also been less generous with their grading than institutions in other geographic regions, and schools that focus on science and engineering tend to be stingier with their A’s than liberal arts schools of equal selectivity.

What accounts for the higher G.P.A.’s over the last few decades?

The authors don’t attribute steep grade inflation to higher-quality or harder-working students. In fact, one recent study found that students spend significantly less time studying today than they did in the past.

Rather, the researchers argue that grade inflation began picking in the 1960s and 1970s probably because professors were reluctant to give students D’s and F’s. After all, poor grades could land young men in Vietnam.

They then attribute the rapid rise in grade inflation in the last couple of decades to a more “consumer-based approach” to education, which they say “has created both external and internal incentives for the faculty to grade more generously.” More generous grading can produce better instructor reviews, for example, and can help students be more competitive candidates for graduate schools and the job market.

The authors argue that grading standards may become even looser in the coming years, making it increasingly more difficult for graduate schools and employers to distinguish between excellent, good and mediocre students.

More disturbing, they argue, are the potential effects on educational outcomes.

“When college students perceive that the average grade in a class will be an A, they do not try to excel,” they write. “It is likely that the decline in student study hours, student engagement, and literacy are partly the result of diminished academic expectations.”