What prompts women to swap good jobs for days spent watching "Dora the Explorer," stirring mac 'n' cheese and mediating play-date squabbles?

Two Macalester College professors decided to ask them.

Karine Moe, an economist, and Dianna Shandy, an anthropologist, took an unusual approach, combining their two disciplines' methods to examine the big picture sketched by statistics as well as the complex details of individual lives. They surveyed 857 women who had graduated from Macalester between 1970 and 2006 as well as hundreds nationwide for intense, open-ended questioning.

In their new book, "Glass Ceilings and 100-Hour Couples: What the Opt-Out Phenomenon Can Teach Us About Work and Family," Moe and Shandy describe what they learned about why some women were motivated to "opt out" of the work force. They came to see the decision as a reaction to never-ending work, family and cultural pressures.

"For every woman who gets opt-out status, there's a really big number of women reducing or reorienting their career because of family," said Shandy who, like Moe, has two children.

These career sacrifices go a long way toward explaining the gender wage gap: Although women on average make less than 80 cents for every dollar earned by men, much of that disparity occurs among employees with kids.

Women who quit or scale back their jobs forfeit not just earnings but also retirement benefits and promotion opportunities. The loss of a spouse's income as a result of a layoff, divorce or death can jeopardize their long-term financial security. Stay-at-home mothers often face big challenges in trying to reenter the work force. The Center for Work-Life Policy in New York found that 25 percent of highly qualified women attempting to return to work were unable to find jobs, and this was in 2004 when the economy was much stronger.

"Employers question gaps in your résumé, technology changes, your skills become obsolete," Moe said.

Even when they do get jobs, women who have opted out often are paid less than they were making before they quit. Women who take time off suffer an 18 percent wage penalty, not including lost retirement contributions, according to the Work-Life Policy study. For women whose break lasts three years or more, the number rises to 37 percent.

"These aren't just individual issues -- as a society, we haven't yet figured out how we need to accommodate parents," Shandy said. "I'm still kind of caught with a sense of amazement that after all these years, women still struggle so much to combine paid employment and motherhood."

More than three-quarters of married mothers with children under 15 work outside the home, but the Macalester professors were interested in going deeper than the statistics. Shandy and Moe questioned their subjects about, among other things, their work patterns. They asked women whether they had ever taken time off, turned down promotions, rejected assignments involving travel, or switched from jobs with a lot of responsibility to ones with fewer demands.

A more nuanced picture emerged. Among women who continued to draw a paycheck, Moe said, "many had reduced their work hours at some point."

To some observers, the choice to quit work is mainly a matter of maternal devotion -- these women feel their children need them at home, and consider that more important than their own professional and financial success.

Others counter that women leave work reluctantly, frustrated after trying to balance parenting with job structures that date to when most workers had wives at home taking care of the house and kids.

Moe and Shandy found that mothers opt out for these reasons and more. Good child care can be hard to find, especially services whose hours mesh with long workdays. Women still shoulder most of the housework and child care. Men do more than they used to, but according to the Bureau of Labor Statistics, married mothers of young children still do two-thirds of household work.

Also in the mid- to late '90s, child-rearing trends swung toward an increasingly intensive, time-consuming approach. Parents came to believe that "their children's lives needed to be closely managed," Moe said.

"There'd be two college-educated folks, and even if they were earning similar incomes, the women would say, 'You know what? It should be me. I might as well stay home because that's the way the world is set up; that's how our culture has things set up,'" Shandy said.

Katy Read is a Minneapolis freelance writer.