Ashenfelter and Hannan (1986)
Ashenfelter, Orley, and Timothy Hannan. "Sex Discrimination and Product Market Competition: The Case of the Banking Industry." Quarterly Journal of Economics, February 1986.
Reviews 120 banks in the Philadelphia region for the year 1976 (selected because it was pre-Affirmative Action), and finds a statistically significant relationship between market concentration and employment of women. Lengthy review of the literature cites numerous other studies indicating the same relationship.