Women were long inadvertent but key drivers of social mobility. Marrying the boss was one way for the secretary to escape her social background and lift her offspring into a higher stratum of income, networks and cultural sophistication.

As women have overtaken men in education and are catching up with them in the job market, the rise of what sociologists and economists call assortative mating — people picking spouses with similar educational achievements and incomes — has been pronounced.

Today, across the member countries of the O.E.C.D., 40 percent of couples in which both partners work belong to the same or a neighboring earnings bracket, compared with 33 percent two decades ago, a 2011 report by the agency shows. Nearly two-thirds of couples have the same level of educational attainment (in 15 percent of the cases, the wife is more educated than her husband).

Doctors used to marry nurses. Now doctors marry doctors.