Women on the senior management track used to ask me what the big secret was to get on boards of public companies.

Years later, women still want to know. Apparently, it's not only still difficult for a woman to get on a board, but it's happening even less often.

The nonprofit research group Catalyst says that in 2006, women held 14.6 percent of board seats at Fortune 500 companies — down from 14.7 percent in 2005. Women of color held 3.1 percent of all Fortune 500 board seats, down from 3.4 percent in 2005. And the number of companies with no women board directors increased to 58 in 2006, from 53 in 2005.

You'd think two factors would have had the opposite effect. One is the growing pressure to have more board diversity. Even back in 1986, shareholders at Wal-Mart and Helen Walton, wife of company founder Sam Walton, were pushing to appoint a woman to the company's 15-member board of directors, according to a recent New York Times article. That's when Hillary Rodham Clinton began her six-year tenure on the board of Wal-Mart, the country's largest company.

The other factor is the increased accountability of a board of directors as a result of the Sarbanes-Oxley Act requiring more corporate governance. Since the law took effect in 2002, chief executive officers — the natural candidates for corporate board positions — have been turning down board jobs in droves, an MIT Sloan Management Review article showed.

The article cites several studies that show a 30 percent decline from 2002 to 2006 in the number of Fortune 500 outside directorships held by chief executives. Another study found a 38 percent drop in new outside CEO directors in companies in the Standard & Poors 500-stock index. It's not surprising, the article says, because of the personal liability and professional embarrassment a board member can face if a company doesn't properly monitor internal systems.

So if not chief executives, who is getting tapped for board openings?

Not women, Cheryl Francis writes on Forbes.com. One reason may be because "women are even more scarcely represented among retired CEOs — the second most popular source of new board members — than among sitting CEOs."

Another group getting the board positions is other senior managers such as chief financial officers and division presidents. The MIT article speculates that they're motivated to take the jobs because "experience as a director can help open new career paths and offer networking possibilities — possibly even leading to a CEO position" — the same reasons women tell me they want those positions.

I often hear senior executives say they don't know any qualified women to serve on the board.

Perhaps companies need to be more imaginative and flexible about recruiting women if — as Francis writes — "they are serious about tapping this reservoir of talent. You can't rely solely on the people whom current board members know and recommend."

Sam Walton appeared relieved to have a woman on the board to deflect shareholder criticism, The New York Times article says. There are plenty of other reasons to have more women on boards.

Women are a dominant force in business and purchasing power, making up 83 percent of all consumer purchases, Marty Barletta says in her book, "Marketing To Women."

Women hold nearly 50 percent of equities and make up more than 50 percent of the educated work force, Francis says.

The value of a woman's perspective, talents and expertise is clear. To be asked to join a board is an honor more qualified women deserve and from which more companies would prosper.