A professor friend of mine recently noticed something in her department: all of the tenure-track women (except her) had stopped their tenure clocks for various reasons: e.g., for childbirth or for issues involving school-aged children with disabilities. All of these women used their unclocked years very 'productively', according to my friend, and ended up with more publications and grants listed in their promotion and tenure files than did my (single, childless) friend. My friend was feeling stressed out about her publication and funding record in comparison to the records of these colleagues.
Last summer, I wrote about an issue that is sort of related to this: whether or not someone has a "baby gap" in their CV owing to time away from research, most likely related to having a baby. In the ensuing discussion, it was clear that some of us who are fortunate enough to have a healthy baby and be healthy ourselves may use maternity/paternity leave or stopped-tenure-clock-time to get additional research done. I don't think anyone would deliberately procreate as a way to get ahead in their publication record (a strategy that has so many negative aspects to it, I shall not comment on it further), but if you find yourself able to get some work done at a time when you supposedly are not able to get much done, why not do so?
Ideally, universities and colleges will have the resources to give all tenure-track faculty a pre-tenure research leave of a term or more, or, at the very least, a reduced teaching load for a while. If this is not possible, presumably the expectations for tenure and promotion are adjusted accordingly. When I write a letter for someone's tenure and promotion review, I always look at their teaching load and use that as one element of my determination of whether their research quality and quantity seems reasonable.
I can see why my friend is stressed out about her situation and why she is comparing herself to her colleagues in this way, but I think that any productivity boost that comes from an unclocked year taken for family or medical reasons is unlikely to be so immense that it creates an uneven playing field (<-- sports reference!).
There are many factors that determine how many publications and grants one ends up with when the P&T file is submitted: professional factors and life factors, some of which are under our control and some of which are not. I think it is unwise to feel hostile towards those who stop the tenure clock, as if they have an unfair advantage.
Feeling that clock-stoppers have an unfair advantage is sort of like feeling that women in general have an unfair advantage -- "They had to hire a woman" is similar to "She wouldn't have gotten tenure if she hadn't stopped the tenure clock for a year". Although I chose not to stop my tenure clock, I strongly believe that tenure clock stoppage is something that the academic community as a whole should embrace as a way of making academe more appealing to a more diverse range of young faculty.
11 years ago