Update: Stuart Rojstaczer, the co-author of the grade inflation study summarized below, is taking reader questions. Submit them here.

Over the last 50 years, college grade-point averages have risen about 0.1 points per decade, with private schools fueling the most grade inflation, a recent study finds.

The study, by Stuart Rojstaczer and Christopher Healy, uses historical data from 80 four-year colleges and universities. It finds that G.P.A.’s have risen from a national average of 2.52 in the 1950s to about 3.11 by the middle of the last decade.

For the first half of the 20th century, grading at private schools and public schools rose more or less in tandem. But starting in the 1950s, grading at public and private schools began to diverge. Students at private schools started receiving significantly higher grades than those received by their equally-qualified peers — based on SAT scores and other measures — at public schools.

In other words, both categories of schools inflated their grades, but private schools inflated their grades more. The chart below shows average G.P.A.’s from 1930 to 2006. Gray dots represent individual schools’ average G.P.A’s. The blue and green lines represent the average G.P.A. for each school type — public or private — over time:

Based on contemporary grading data the authors collected from 160 schools, the average G.P.A. at private colleges and universities today is 3.3. At public schools, it is 3.0.

The authors suggest that these laxer grading standards may help explain why private school students are over-represented in top medical, business and law schools and certain Ph.D. programs: Admissions officers are fooled by private school students’ especially inflated grades.

Additionally, the study found, science departments today grade on average 0.4 points lower than humanities departments, and 0.2 points lower than social science departments. Such harsher grading for the sciences appears to have existed for at least 40 years, and perhaps much longer.

Relatively lower grades in the sciences discourage American students from studying such disciplines, the authors argue.

“Partly because of our current ad hoc grading system, it is not surprising that

the U.S. has to rely heavily upon foreign-born graduate students for technical fields of research and upon foreign-born employees in its technology firms,” they write.

These overall trends, if not the specific numbers, are no surprise to anyone who has followed debates about grade inflation. But so long as schools believe that granting higher grades gives alumni an advantage, there will be little or no incentive to impose stricter grading standards unilaterally.

You can find a database of G.P.A.’s by school over the last few years (or in some cases, the last few decades) here.