Why don't women run the world? Maybe they don't want to.

But to talk to the women of the book club -- or to the women of a San Francisco mothers' group with whom I also spent time, or the dozens of other women I interviewed, or the countless women I have come to know during the four years I have reported on the intersection of life and work -- is to sense that something more is happening here. It's not just that the workplace has failed women. It is also that women are rejecting the workplace.

I say this with the full understanding that there are ambitious, achieving women out there who are the emotional and professional equals of any man, and that there are also women who stayed the course, climbed the work ladder without pause and were thwarted by lingering double standards and chauvinism. I also say this knowing that to suggest that women work differently than men -- that they leave more easily and find other parts of life more fulfilling -- is a dangerous and loaded statement.

And lastly, I am very aware that, for the moment, this is true mostly of elite, successful women who can afford real choice -- who have partners with substantial salaries and health insurance -- making it easy to dismiss them as exceptions. To that I would argue that these are the very women who were supposed to be the professional equals of men right now, so the fact that so many are choosing otherwise is explosive.

Don't look at her as something out of ''The Bell Jar'' either. She is not trapped. This is a choice. And don't worry for her that she will have no resources should something happen to her spouse, his career or their marriage, she insists. ''My degree is my insurance policy.''

But is it enough insurance? Not only in the event that she needs to go back to work, but also when the time comes, that she wants to. Because at the moment, it is unclear what women like these will be able to go back to. This is the hot button of the work-life debate at the moment, a question on which the future of women and work might well hinge. For all the change happening in the office, the challenge of returning workers -- those who opted out completely, and those who ratcheted back -- is barely even starting to be addressed.

If that workplace can reabsorb those who left into a career they find fulfilling, then stepping out may in fact be the answer to the frustrations of this generation. If not, then their ability to balance life and work will be no different than their mothers', after all.

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But Hewlett's preliminary research makes her pessimistic about what today's women will face when they want to return to work. At any given time, she says, ''two-thirds of all women who quit their career to raise children'' are ''seeking to re-enter professional life and finding it exceedingly difficult. These women may think they can get back in,'' she said, when I told her of what I had been hearing in San Francisco and Atlanta and on my own suburban street, where half the women with children at home are not working and where the jobs they quit include law partner and investment banker. ''But my data show that it's harder than they anticipate. Are they going to live to the age of 83 and realize that they opted out of a career?''

Tilghman is now a leader. In that role she wonders how to educate women to enter this shades-of-gray world and how to create an environment for her own staff that encourages a balanced life. But Tilghman is also a scientist, and she suspects that policies and committees, while crucially important, cannot change everything. And she wonders whether evolution has done both men and women a disservice.

''My fantasy is a world where there are two kinds of people -- ones who like to stay home and care for children and ones who like to go out and have a career,'' she says. ''In this fantasy, one of these kinds can only marry the other.'' But the way it seems to work now is that ambitious women seem to be attracted to ambitious men. Then when they have children together, ''someone has to become less ambitious.'' And right now, it tends to be the woman who makes that choice.

She compares all this to a romance gone sour. ''Timing one's quitting to coincide with a baby is like timing a breakup to coincide with graduation,'' she says. ''It's just a whole lot easier than breaking up in the middle of senior year.''

That is the gift biology gives women, she says. It provides pauses, in the form of pregnancy and childbirth, that men do not have. And as the workplace becomes more stressful and all-consuming, the exit door is more attractive. ''Women get to look around every few years and say, 'Is this still what I want to be doing?''' she says. ''Maybe they have higher standards for job satisfaction because there is always the option of being their child's primary caregiver. When a man gets that dissatisfied with his job, he has to stick it out.''

This, I would argue, is why the workplace needs women. Not just because they are 50 percent of the talent pool, but for the very fact that they are more willing to leave than men. That, in turn, makes employers work harder to keep them. It is why the accounting firm Deloitte & Touche has more than doubled the number of employees on flexible work schedules over the past decade and more than quintupled the number of female partners and directors (to 567, from 97) in the same period. It is why I.B.M. employees can request up to 156 weeks of job-protected family time off. It is why Hamot Medical Center in Erie, Pa., hired a husband and wife to fill one neonatology job, with a shared salary and shared health insurance, then let them decide who stays home and who comes to the hospital on any given day. It is why, everywhere you look, workers are doing their work in untraditional ways.

Women started this conversation about life and work -- a conversation that is slowly coming to include men. Sanity, balance and a new definition of success, it seems, just might be contagious. And instead of women being forced to act like men, men are being freed to act like women. Because women are willing to leave, men are more willing to leave, too -- the number of married men who are full-time caregivers to their children has increased 18 percent. Because women are willing to leave, 46 percent of the employees taking parental leave at Ernst & Young last year were men.

Looked at that way, this is not the failure of a revolution, but the start of a new one. It is about a door opened but a crack by women that could usher in a new environment for us all.

Many women do not have a choice. They are single, divorced, married to a man who doesn't make enough to support the family alone...