"Professors say that the only aspect of their teaching that matters professionally is student course evaluations, since these can figure in tenure and promotion decisions. It’s in professors’ interest, therefore, for their classes to be entertaining and their assignments not too onerous. They are not deluded: a study carried out back in the nineteen-nineties (by Alexander Astin, as it happens) found that faculty commitment to teaching is negatively correlated with compensation."

Ah, thanks. I went back and read it two more times and didn't find it, but there it is. (For any one else looking for this excerpt, it's mentioned in passing mid-paragraph half way down page 3.)