From The Wall Street Journal:
… Republicans tolerated inequality, handing out more very high and very low grades, while Democrats' grades grouped more tightly around the average. Republicans also gave black students lower grades than did their colleagues.
The study is (still) forthcoming out in American Economic Journal: Applied Economics. Kudos to the newspaper: I went and read the journal piece second, and didn’t get much extra out of it.
It is not capable of determining if Republicans give lower grades to blacks because they are prejudiced, or because Democrats are giving them better grades than they deserve, or if blacks have weaker backgrounds.
But, the study finds no gender effects, or racial effects other than the Republican-professor-black-student one.