The waters of Lake Wobegon have flooded U.S. college campuses. A’s — once reserved for recognizing excellence and distinction — are today the most commonly awarded grades in America.

That’s true at both Ivy League institutions and community colleges, at huge flagship publics and tiny liberal arts schools, and in English, ethnic studies and engineering departments alike. Across the country, wherever and whatever they study, mediocre students are increasingly likely to receive supposedly superlative grades.

Such is the takeaway of a massive new report on grade inflation from Stuart Rojstaczer, a former Duke University professor, using data he and Furman University professor Chris Healy collected. Analyzing 70 years of transcript records from more than 400 schools, the researchers found that the share of A grades has tripled, from just 15 percent of grades in 1940 to 45 percent in 2013. At private schools, A’s account for nearly a majority of grades awarded.

These findings raise questions not only about whether the United States has been watering down its educational standards — and hampering the ability of students to compete in the global marketplace in the process. They also lend credence to the perception that campuses leave their students coddled, pampered and unchallenged, awarding them trophies just for showing up.

So, what’s behind the sharp rise in GPAs?

Students sometimes argue that their talents have improved so dramatically that they are deserving of higher grades. Past studies, however, have found little evidence of this.

While it’s true that top schools have become more selective, the overall universe of students attending college has gotten broader, reflecting a wider distribution of abilities and levels of preparation, especially at the bottom. College students today also study less and do not appear to be more literate than their predecessors were.

Plus, of course, even if students have gotten smarter, or at least more efficient at studying (hey, computers do help), grades are arguably also supposed to measure relative achievement among classmates.

Affirmative action also sometimes gets blamed for rising grades; supposedly, professors have been loath to hurt the feelings of underprepared minority students. Rojstaczer and Healy note, however, that much of the increase in minority enrollment occurred from the mid-1970s to mid-’80s, the only period in recent decades when average GPAs fell.

Rapid inflation instead occurred during two distinct periods: first from the 1960s through the early 1970s, then during the mid-1980s through today.

That first era, the researchers say, can be explained by changes in pedagogical philosophy (some professors began seeing grades as overly authoritarian and ineffective at motivating students) and mortal exigencies (male students needed higher grades to avoid the Vietnam draft).

The authors attribute today’s inflation to the consumerization of higher education. That is, students pay more in tuition, and expect more in return — better service, better facilities and better grades. Or at least a leg up in employment and graduate school admissions through stronger transcripts.

And indeed, some universities have explicitly lifted their grading curves (sometimes retroactively) to make graduates more competitive in the job market, leading to a sort of grade inflation arms race.

But rising tuition may not be the sole driver of students’ expectations for better grades, given that high school grades have also risen in recent decades. And rather than some top-down directive from administrators, grade inflation also seems related to a steady creep of pressure on professors to give higher grades in exchange for better teaching evaluations.

It’s unclear how the clustering of grades near the top is affecting student effort. But it certainly makes it harder to accurately measure how much students have learned. It also makes it more challenging for grad schools and employers to sort the superstars from the also-rans (which, if you’re an elite school like Harvard, is probably a feature, not a bug).

Lax or at least inconsistent grading standards can also distort what students — especially women — choose to study, pushing them away from more stingily graded science, technology, engineering and math fields and into humanities, where high grades are easier to come by.

But what to do about the rise of the Gentleman’s A?

A decade ago my alma mater, Princeton, decided to lead the charge against grade inflation. But when students grumbled, and other schools didn’t follow, the school abruptly abandoned its fight. Wellesley, which had the highest GPA in Rojstaczer’s database in 2000 (3.55), underwent a similar endeavor, only to see its grades creep back up more recently.

Without collective action — which means both standing up to students and publicly shaming other schools into adopting higher standards — the arms race will continue.