Though the study did not analyze fathers’ role in depth, it found that their beliefs did not change significantly before and after having a baby. They were less likely than women to say that parenthood was harder than they expected. (Women still do the bulk of child care, even in two-earner families.)

Women got it so wrong, the researchers argue, because it has become harder to work and have children.

The cost of motherhood fell for most of the 20th century because of inventions like dishwashers, formula and the birth control pill. But that’s no longer the case, according to data cited in the paper. The cost of child care has increased by 65 percent since the early 1980s. Eighty percent of women breast-feed, up from about half. The number of hours that parents spend on child care has risen, especially for college-educated parents, for whom it has doubled.

Over all , women have had great success in entering the labor force. Seventy percent of mothers with children under 18 work. Women are more likely to work than previous generations at almost every age, found Claudia Goldin, a Harvard economist. They’re slightly more likely to stop in their late 30s and early 40s, around the time many are taking care of young children — but they usually return to the labor force, particularly if they have degrees.

Still, the new paper raises questions about why the work-family juggle seems to be getting harder. “It is deeply puzzling that at a moment when women are more prepared than ever for long careers in the labor market, norms would change in a manner that encourages them to spend more time at home,” the researchers wrote.

One possible reason is that increasingly, people who work long, inflexible hours are paid disproportionately more, Ms. Goldin’s research has found. More women with degrees and these kinds of demanding jobs are having children, and they’re likely to be married to men with similar jobs, as Marianne Bertrand, an economist at the University of Chicago, has described. A result is that dual-earning couples may feel the best choice is for one member, usually the mother, to step back from work so the other parent can maximize the family’s earnings.

To try to set their children on the best path amid increased competition for college admission, parents, especially college-educated ones, invest significantly more time than they used to in child care, found Valerie Ramey and Garey Ramey, economists at the University of California, San Diego. They described it as the “rug rat race” for top colleges.