Here is a new paper by Donna K. Ginther and Shulamit Kahn:

This study uses data from Academic Analytics to examine gender differences in promotion to associate professor in economics. We found that women in economics were 15% less likely to be promoted to associate professor after controlling for cumulative publications, citations, grants and grant dollars. In contrast, we found no significant gender differences in promotion in other fields including biomedical science, physical science, political science, mathematics and statistics, and engineering. We separated the sample by the research intensity of institutions and found suggestive evidence that these results were being driven by less research-intensive institutions.

What is the best model for understanding this result?  The "ol' boys' network" matters more at lower-tier institutions?  Something else?  There doesn't seem to be a gender tenure penalty at higher-ranked research institutions.