A member of my university's Board of Regents once asked a group of us who were assembled to explain tenure to them, "Do you have to be liked to get tenure?" The answer is no, of course. You don't have to be a nice person to do great research. But do you have to be a good person, in the sense of not being too much of an unethical jerk (as opposed to just a regular jerk)?
Almost always, I can focus on the science when I am reviewing a paper or a proposal by a regular jerk, and if I don't feel that I can be objective (to the extent that one should try to be, anyway), I don't do the review. Right now, I'm torn about whether to review a proposal that was sent to me recently. I think that the PI's department and the students and researchers will benefit greatly from the equipment that is part of this proposal, and the PI is probably just a figurehead senior guy who was available to put his name on the cover page.
A review of the proposal should therefore not focus too much on the fact that he hasn't published in decades and hasn't had an original idea since 1973, or on the fact that he is a flaming jerk, serial molester of students, and currently involved in an affair with one of his students (formerly an undergrad in his department, now his grad student). She is 48 years younger than he is, and her research project is, unfortunately, a sham (and is described in the proposal). OK, I think I just decided that I can't be objective about this one. I hope that other reviewers will overlook the unproductive jerk PI and be positive enough that the students (other than his girlfriend/student) and others in his department will get the benefit of the grant.
9 years ago