The corporate quest for equality for women in management is a lot like motherhood: Everybody is for it. Bain & Company recently conducted a survey of more than 1,800 businesspeople worldwide, and nearly 80% of them—women and men—said they were convinced of the benefits of gender parity at all levels. And for good reason: Many businesses recognize that retaining more women as they ascend the corporate ladder will add diversity of experience and perspective and also will help them understand women as buyers and influencers. Higher retention rates will also save companies millions in recruiting and retraining costs. There's only one problem: The mechanism for getting women into leadership positions is flawed.

Do the math: Women constitute 50% of the workforce in America, but they represented only 3% of the chief executives of the country's 500 largest companies in 2009. The female-to-male ratio rapidly dwindles at almost every rung of the ladder upward, across organizations and across industries. It's the biggest disappearing act on earth, and it arises from two significant blind spots that most companies seem to have.

The first has to do, literally, with motherhood. Companies have responded to parents' demands for more flexibility by letting them go part-time or take time off to raise their children, but they haven't figured out how to bring them back onto a viable career path when they're ready to return to the workforce. Our survey results reveal that women tend to make many more compromises to their career paths than do their male partners. They are nearly twice as likely to take a flexible career path or a leave of absence and three times as likely to work part-time. The majority of promotion processes and career paths thus have a built-in biological bias, linked to the time women take off for having and rearing children. Some estimates show that more than 90% of women want to return but only 40% can find full-time jobs.

The reality is that in any group of equally competent and talented men and women of the same tenure, women who have taken time off or worked part-time for family reasons lack equal experience, by definition. That matters a lot when they are considered for promotion. Result: Men usually get the job. Ambitious and talented women then face a choice of either giving up or getting out. It's no surprise that women entrepreneurs start nearly 1,600 businesses daily in the U.S. One woman who traded in her corporate career told us in the survey: "I chose to leave the corporate world and run my own company rather than try to achieve gender parity at that corporation, in that industry."

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