Suppose you could memorize only a single demographic number and you set about choosing the one with the most far-reaching implications for change in America. You could do worse than 1.5.

Of course, there are plenty of possibilities: the birth rate, the teen-pregnancy or illegitimacy rate, the percentage of the population that is white or foreign-born, the percentage of elderly. But unpack 1.5 and you have the makings of a social inversion: a turning upside down of the male-dominated order that Americans have taken for granted since—well, since forever.

The number 1.5 is, in this case, a ratio. According to projections by the National Center for Education Statistics, in 2017 half again as many women as men will earn bachelor's degrees. In the early 1990s, six women graduated from college for every five men who did so; today, the ratio is about 4-to-3. A decade from now, it will be 3-to-2—and rising, on current trends.

What does this mean? And what's going on? Neither question is easy to answer. But start with the second.

A college degree used to be a rarity: a mark of privileged or professional status. As recently as 1950, fewer than half of Americans even finished high school, let alone went on to college.

Surprisingly, in the early decades of the last century, college attendees were as likely to be female as male. As the economists Claudia Goldin, Lawrence F. Katz, and Ilyana Kuziemko note in a fascinating 2006 article in the Journal of Economic Perspectives, things changed dramatically beginning in the 1930s. Men poured into universities, first to escape Depression-era unemployment, later with the help of the G.I. Bill, then to escape Vietnam. Above all, men were responding rationally to a labor market that paid a rising premium for advanced education. By 1957, three men took home a college diploma for every two women who did.

That imbalance defined the world in which all but the youngest of today's adults grew up. The education gap bolstered the presumption that men would dominate the professions and other elite careers; that men would boss women, instead of the other way around; that men, with their college-turbocharged earning power, would be the primary breadwinners; that, educationally speaking, men could expect to marry down.

Chapter 3 of the 20th-century story is as welcome as it is well known. Feminism, family planning (in the form of birth control, especially the Pill), and a meritocratic labor market opened not just jobs but careers to women, who streamed into the workforce and formed two-earner families. Expecting to work—and also, as divorce rates soared, worrying about having to support themselves—women also streamed to college. By about 1980, the gender gap in college enrollment had vanished. Young women had reached educational parity, with the promise of social parity not far behind.

The puzzle is what happened next. In the 1990s, the pattern changed again, but the surprise involved men. The wage premium for a college degree continued to rise smartly. Women responded just as economic theory predicts that rational actors would: Their college attendance rates kept climbing because the more they learned, the more they earned.

Men, however, ignored what the market was telling them: Their college attendance and completion rates barely rose. Why? "That's the big mystery," says Gary Burtless, an economist at the Brookings Institution.

Whatever the reason, the result was a new educational gender gap, this time favoring women. There is little sign that it will close: Projections by the National Center for Education Statistics show a 22 percent increase in female college enrollment between 2005 and 2016, compared with only a 10 percent increase for men.

In 2006, according to the Census Bureau, about 27 million American men held a college degree; so did about 27 million American women. This is a tipping point, however, not an equilibrium, because male college graduates tend to be old, and female graduates tend to be young. Among people age 65 and older, men are much more likely than women to be college-educated. Middle-aged men and women are at parity. Among young adults ages 25 to 34 years old, the college gap favors women almost as lopsidedly as it favors men among their grandparents' generation.

In other words, today's young people already live in a world where, among their peers, women are better educated than men. As the grandparents die off, every year the country's college-educated population will become more feminized. In a couple of decades, America's educational elite will be as disproportionately female as it once was male.

Perhaps men will wake up, smell the coffee, and rush off to college in greater numbers. Or perhaps the labor market will undergo a sea change and the premium on education will stop rising and start falling. As of now, however, both of those reversals appear far-fetched. Men might—certainly should, and hopefully will—raise their college attendance rates, but the likely effect would be to narrow the gap with women, not close it, much less flip it.

Meanwhile, millions of semiskilled workers in developing countries are entering an increasingly globalized labor market, which all but guarantees a rise in the relative premium commanded by a college diploma.

So what we are talking about, in all likelihood, is an America where women are better educated than men and where education matters more than ever. Put those facts together, and you get some implications worth pondering.

In 1978, when I was a freshman in college, I met a woman who told me she was in law. "Oh," I said, "you're a secretary?" Her gentle but mortifying reply: "No, I'm a lawyer." Few of today's young people can even imagine making that kind of faux pas. According to census data, a higher share of women than men already work in management and professional jobs (37 percent versus 31 percent, in 2005).

Look for that gap to widen. A generation from now, the female lawyer with her male assistant will be the cliché. Look for women to outnumber men in many elite professions, and potentially in the political system that the professions feed. (The election of a female president is a question of when, not whether.)

Women's superior education will increase their earning power relative to men's, and on average they will be marrying down, educationally speaking. A third of today's college-bound 12-year-old girls can expect to "settle" for a mate without a university diploma. But women will not stop wanting to be hands-on moms.

For families, this will pose a dilemma. Women will have a comparative advantage at both parenting and breadwinning. Many women will want to take time off for child-rearing, but the cost of keeping a college-educated mom at home while a high-school-educated dad works will be high, often prohibitive.

Look, then, for rising pressure on government to provide new parental subsidies and child care programs, and on employers to provide more flextime and home-office options—among various efforts to help women do it all. Look, too, for a cascading series of psychological and emotional adjustments as American society tilts, for the first time, toward matriarchy. What happens to male self-esteem when men are No. 2 (and not necessarily trying harder)? When more men work for women than the other way around?

Some of these adjustments will have international dimensions. Goldin, Katz, and Kuziemko note, "Almost all countries in the OECD"—the Organization for Economic Cooperation and Development, a group of advanced industrial countries—"now have more women than men in college and have had a growing gender gap among undergraduates that favors women." Yet much of the developing world, especially the Muslim world, remains predominantly patriarchal.

Many tradition-minded cultures in the Middle East, Africa, and parts of Asia already regard the Western economic and social model as emasculating. Radical Islam, in particular, abhors feminism. As the United States and Europe continue to feminize, will the anti-modern backlash, already deeply problematic in the Muslim world, intensify? As sex roles and expectations diverge, might hostility and misunderstanding mount between the West and the rest?

No, men are not about to disappear into underclass status. They will not become mothers anytime soon, and they will not stop secreting testosterone. Men's ambition will ensure ample male representation at the very top of the social order, where CEOs, senators, Nobelists, and software wunderkinds dwell. Women will not rule men.