Throughout the 1980s and early '90s, women of all economic levels - poor, middle-class and rich - steadily gained on their male counterparts in the work force. By the mid-1990s, women earned more than 75 cents for every dollar in hourly pay that men did, up from 65 cents 15 years earlier.
Largely without notice, however, one big group of women has stopped making progress: those with a four-year college degree. The gap between their pay and that of male college graduates has widened slightly since the mid-'90s.
For women without a college education, the pay gap with men has narrowed slightly over the same span.
A decade ago, it was possible to imagine that men and women with similar qualifications might soon be making nearly identical salaries. Today, that is far harder to envision.
"Nothing happened to the pay gap from the mid-1950s to the late '70s," said Francine D. Blau, an economist at Cornell University and a leading researcher of sex and pay. "Then the '80s stood out as a period of sharp increases in women's pay. And it's much less impressive after that."
Last year, for example, college-educated women age 36 to 45 earned 74.7 cents in hourly pay for every dollar that men in the same group did, according to Labor Department data analyzed by the Economic Policy Institute. A decade earlier, the women earned 75.7 cents.
The reasons for the stagnation are complicated and appear to include both discrimination and women's choices. The number of women staying home with young children has risen recently, according to the Labor Department; the increase has been sharpest among highly educated mothers, who might otherwise be earning high salaries. The pace at which women are flowing into highly paid fields also appears to have slowed.
Like so much about sex and the workplace, there are at least two ways to view these trends. One is that women, faced with most of the burden for taking care of families, are forced to choose jobs that pay less - or, in the case of stay-at-home mothers, pay nothing at all.
If the government offered day care programs similar to those in other countries or if men spent more time caring for family members, women would have greater opportunity to pursue whatever job they wanted, according to this view.
The other view is that women consider money a priority less often than men do. Many relish the chance to care for children or parents and prefer jobs, like those in the nonprofit sector, that enable them to influence other people's lives.
Both views, economists say, could contain some truth.
"Is equality of income what we really want?" asked Claudia Goldin, an economist at Harvard University who has written about the revolution in women's work over the past generation. "Do we want everyone to have an equal chance to work 80 hours in their prime reproductive years? Yes, but we don't expect them to take that chance equally often."
Whatever role their own preferences might play in the pay gap, many women say they continue to battle subtle forms of lingering prejudice.
Indeed, the pay gap between men and women who have similar qualifications and work in the same occupation - which economists say is one of the purest measures of sex equality - has barely budged since 1990. Today, the discrimination often comes from bosses who believe they treat everyone equally, women say, but it can still create a glass ceiling that keeps them from reaching the best jobs.
Economists say the recent pay trends have been overlooked because the overall pay gap, as measured by the government, continues to narrow.
The average hourly pay of all female workers rose to 80.1 percent of men's pay last year, from 77.3 percent in 2000. But that is largely because women continue to close the qualifications gap. More women than men now graduate from college, and the number of women with decades of work experience is growing rapidly.
Within many demographic groups, though, women are no longer gaining ground.
Blau and her husband, Lawrence M. Kahn, another Cornell economist, have done detailed studies of sex and pay, comparing men and women who have the same occupation, education, experience, race and labor-union status. By the late 1970s, women earned about 82 percent as much as men with similar profiles. A decade later, the number had shot up to 91 percent - raising the question whether women would reach parity.
But by the late '90s, the number remained at 91 percent. Blau and Kahn have not examined the current decade in detail, but Blau said other data suggested there had been little movement.
There is no proof that discrimination is the cause of the remaining gap, Blau said. It is possible that the average man, brought up to view himself as the main breadwinner, is more committed to his job than the average woman.
But researchers note that government efforts to reduce sex discrimination have ebbed over the period that the pay gap has stagnated. In the 1960s and '70s, laws like Title VII and Title IX prohibited discrimination at work and in school and may have helped close the pay gap. There have been no similar pushes in the past couple of decades.