A new glass ceiling in the boardroom? Representation of women on corporate boards seems to be stagnating. Here's why.

• Author: Valenti, Alix
• Position: GUEST COLUMN

DURING THE last 15 years, the number of women on the boards of U.S. companies has increased significantly. According to Catalyst, a nonprofit organization that tracks women in business, women held only 8.3 percent of the total directorships of Fortune 500 companies in 1993. By 2005, the percentage increased to 14.7. However, the data indicate that incremental gains are decreasing. Catalyst itself describes the current advances that women are making in the boardroom as "stagnant." Moreover, smaller firms have been less aggressive in appointing women to their boards, with only 8.9 percent of board seats occupied by women in companies ranked 501 to 1,000.

To determine whether there is any observable pattern of women being appointed to boards and discover why the number of women appointed to boards does not appear to be increasing, I randomly selected 120 of the Fortune 500 companies over a five-year period from 2001 to 2005. Of the 120 companies included in the study, 112 (93.3 percent) had at least one woman on their boards in 2005. The largest number of female members on any one board was five, representing 42 percent of the board seats of Estee Lauder and 50 percent of the directors of Albertson's.

A review of each company's proxy statement for 2001 and 2005 revealed that there were a total of 88 appointments of female directors in those two years. Using a logistical regression procedure, I found that companies were 22 percent more likely to appoint a woman to the board when another female member vacated the board during either the current or prior year. If, however, there were one or more women on the board of directors, the likelihood of appointing another female board member decreased significantly whether or not a vacancy had occurred within the previous 12 months.

These results confirm the view that corporations, at least those among the largest U.S. firms, are concerned that their boards contain a mix of directors that signals to stakeholders desirable and expected corporate objectives. When a woman leaves the board, there is a strong probability that she will be replaced by another female director.

On the other hand, by not appointing a second or third woman when at least one other woman is a member of the board, the company implies that as long as there is at least one female director on its board, it believes it is complying with existing norms regarding the appointment of women as directors--thus explaining why representation of...