One study claims that tenure-track professors tend to be worse teachers (at the introductory level) than adjuncts or other non-tenure-track instructors. See the write-up in the Atlantic here. The main argument: Students taught by non-TT professors are more likely to choose to take a second class in that field, and these students end up doing better in those other classes than students who took introductory courses with TT-professors.

Incidentally, my university, an R1 RU/VH, gives clear percentages for how performance in different aspects of the job affects our merit increases: research 60%, teaching 30%, service 10%.

It’s worth thinking about whether more universities should move toward an Australian National University model, in which there are separate research and teaching faculty.

EDIT: Aeon Skoble has a good point: This is study is just looking at one R1 RU/VH institution. It would be interesting to see whether you’d get similar results at other kinds of institutions. I’d guess that the TT professors at a typical state liberal arts college are better than the adjuncts.