Are you in a small department at a college or university? Are there no women on the physics faculty with professorial rank—no assistant, associate, or full professors? On its face, this might look like discrimination, but it probably isn't, according to a report issued earlier this month by the American Institute of Physics (AIP).

The report's authors, statistician Susan White and sociologist Rachel Ivie, prepared a statistical model of the distribution of gender on faculties of various sizes, assuming no gender bias beyond the general gender disparity in physics. The model aligned well with current reality. With so few women overall on physics faculties—currently about 13% of physics faculty are women—it is to be expected that quite a few smaller departments will have no female members, they conclude.

Physics departments that grant only bachelor's degrees are generally small; the mean number of professorial-rank faculty in such departments is four. Statistically, the report notes, this means that almost half of these departments are expected to be single-sex. According to AIP's 2010 Academic Workforce Survey, 1% of bachelor's only–granting departments have only women with rank of at least assistant professor, and 47% of bachelor's only–granting departments have only men.

Because Ph.D.-granting departments are considerably larger on average—the median number of faculty in such departments is 22—the proportion of those that have faculties consisting of a single sex is expected to be smaller; the model predicts that about 12% of Ph.D.-granting departments will have same-sex faculties. The actual number of womanless departments is smaller: According to the survey, 8% of faculties are single-sex, all of them all-male.

Of course, this doesn't mean there is no gender discrimination in physics; there's still the initial gender imbalance to explain. "We do not mean to imply that the low representation of women is benign," writes Rachel Ivie in an e-mail to Science Careers. "The low representation of women in physics starts much earlier than at the point of hire for physics departments as a whole. It probably occurs even before people take physics in high school, where 50% of students are female. The low representation of women in physics may not be benign (my personal opinion), but it is explainable at the higher levels of academe. This is what we were trying to do in the report."

Jim Austin is the editor of Science Careers. @_JimAustin on Twitter