Why do female professors earn less than male professors? Some charge that gender bias is at play, while others insist that once factors such as experience are accounted for, the gaps aren't consequential.
There may be truth to both views, according to research findings  presented this week at the annual meeting of the American Educational Research Association by Paul D. Umbach, an assistant professor of higher education at the University of Iowa. Umbach used a series of databases to calculate the gender gap in pay over all, and then to account for all kinds of factors other than gender bias that may contribute to the salary gap. In the end, he found that looking at those factors decreases the size of the gap, but that it remains meaningful.
Leaving all factors out, the mean salary for women in the professoriate was 21.8 percent less than that for men. Add all the possible explanations and their impact, and the gap shrinks to 6.8 percent.
While that is much smaller than the original figure, Umbach stressed that the gap matters -- especially since it persists after all the other explanations have been accounted for. "It's still substantial and it's still unexplained," he said.
Umbach based his analysis on a dataset of 2,758 faculty members from 79 disciplines who reported information about themselves in the 1999 National Study of Postsecondary Faculty. He focused only on research universities and on full-time, tenured or tenure-track faculty members.
He started off by producing mean salaries for men and women in the various disciplines, and examining the differentials. In 70 of the 79 disciplines, the male mean was higher, in some cases significantly so. For example, the mean differential favoring men was $12,649 in English literature, $24,845 in chemical engineering, and $23,294 in economics. But these comparisons included men and women at all stages in their careers -- so the senior faculty members with higher salaries (and who are more likely to be men) tilt the sample significantly.
So then Umbach ran a series of analyses designed to compensate for that and other factors. Years of seniority were factored in, as were books and articles written, career patents, whether the person was receiving outside support for research, professorial rank, and the general job market in the discipline (based on percentage of new Ph.D.'s who are employed), among other factors. When all of those factors were added, the gap still remained, at 6.8 percent.
There are not clear explanations for the gap, leaving open the possibility that bias is at play, Umbach said. But he said that other parts of his study suggest that the bias may not be a simple preference for men, but may relate to biases based on disciplines and on how faculty members spend their time.
For instance, Umbach found that as the proportion of females in a discipline increases, the mean salaries drop -- for men and women. Another factor that negatively correlates with salaries is the percentage of time spent teaching: The greater a discipline's time spent on teaching, the lower its salaries -- for men and women. The more outside research funding, the higher the salaries.
In one respect, Umbach said, those findings don't suggest bias because male and female faculty members in the discipline are affected equally. But when these figures are coupled with other studies suggesting, for example, that female professors may spend more time on teaching, questions are raised about underlying bias.
"We know that women tend to be employed in disciplines with a lot of other women, in disciplines without as much funded research, in disciplines with more time teaching," he said. "Is the reward structure more male? Are we creating structures that reward men?"