Professors differ on how much their grading should be based on tests, written assignments, labs, class participation and other factors.

But students' looks? Most faculty members would deny that physical appearance is a legitimate criterion in grading. But a study presented Monday at the annual meeting of the American Economic Association finds that -- among similarly qualified female students -- those who are physically attractive earn better grades than others. For male students, there is no significant relationship between attractiveness and grades. And the results hold true whether the faculty member is a man or a woman.

The study was conducted at Metropolitan State University of Denver (an open-enrollment institution with many nontraditional-age students), by two economists there, Rey Hernández-Julián and Christina Peters.

The two economists obtained student identification photographs and had the attractiveness rated, on a scale of 1-10, of all the students. (The researchers recruited people who were not students or faculty members to rate the students' attractiveness.) Then they examined 168,092 course grades awarded to the students, using factors such as ACT scores to control for student academic ability.

For female students, an increase of one standard deviation in attractiveness was associated with a 0.024 increase in grade (on a 4.0 scale).

The attractiveness gap in grades appears to result more from lower grades for less attractive women than from higher grades for the most attractive women. When the researchers divided the women into three groups -- average, more attractive and less attractive -- they found a very small (and not statistically significant) gain for the above average attractiveness women. But for the least attractive third of women, the average course grade was 0.067 grade points below those earned by others, a statistically significant gap.

The researchers then took advantage of Metro State's significant online offerings, which appeal to many students who also take courses in person. Comparing similar groups of students, the study found that the grade punishment for unattractive women disappears in online education. (Male students' lack of an attractiveness factor on grading is the same nonfactor in person or online.) The online success of the less attractive students suggests, the paper says, that their lower grades in in-person classes can't be attributed to some factor that might make them legitimately earn lower grades.

The Metro State researchers are not the first to suggest an apparent link between attractiveness and academic success. The 2013 book Physical Attractiveness and the Accumulation of Social and Human Capital in Adolescence and Young Adulthood: Assets and Distractions (Wiley) found that those who are attractive in high school are more likely than those with just average or below average looks to go on to earn a four-year college degree.

Via email, Hernández-Julián said that he found the results of the Metro State study “troubling.”

He said that there are two possible explanations: “Is it that professors invest more time and energy into the better-looking students, helping them learn more and earn the higher grades? Or do professors simply reward the appearance with higher grades given identical performance? The likely answer, given our growing understanding of the prevalence of implicit biases, is that professors make small adjustments on both of these margins,” he said.

He added that “tools to address the presence of implicit racial bias in policing are becoming increasingly prevalent. Similar tools might be useful in other environments where other implicit biases are prevalent, such as colleges and universities.”