Millennials, generally defined as people born between 1982 and 2000, were supposed to be the generation that forged what has been called “a new national consensus” in favor of gender equality. Indeed, in February the prominent Columbia professor Jeffrey Sachs labeled the 2016 election, where an extremely qualified female candidate lost to a man with a history of disrespecting women, “a blip” on the road to the egalitarian society that will be achieved once millennial voters outnumber their conservative elders.

But the millennial category lumps together everybody from age 17 to 34, a group varied by race, ethnicity, religion, income, education and life experience. Don’t think for a second they are united. As a set of reports released Friday by the Council on Contemporary Families reveals, fewer of the youngest millennials, those aged 18 to 25, support egalitarian family arrangements than did the same age group 20 years earlier.

Using a survey that has monitored the attitudes of high school seniors for nearly 40 years, the sociologists Joanna Pepin and David Cotter find that the proportion of young people holding egalitarian views about gender relationships rose steadily from 1977 to the mid-1990s but has fallen since. In 1994, only 42 percent of high school seniors agreed that the best family was one where the man was the main income earner and the woman took care of the home. But in 2014, 58 percent of seniors said they preferred that arrangement. In 1994, fewer than 30 percent of high school seniors thought “the husband should make all the important decisions in the family.” By 2014, nearly 40 percent subscribed to that premise.

A different survey found a similar trend, in this case concentrated mainly among men. In 1994, 83 percent of young men rejected the superiority of the male-breadwinner family. By 2014 that had fallen to 55 percent. Women’s disagreement fell far less, from 85 percent in 1994 to 72 percent in 2014. Since 1994, young women’s confidence that employed women are just as good mothers as stay-at-home moms has continued to inch up, but young men’s has fallen. In fact, by 2014, men aged 18 to 25 were more traditional than their elders.

Such slippage in support for gender equality may have been a factor in the 2016 election, even though voters 18 to 30 were more likely than any other age group to vote for Hillary Clinton. An analysis of exit polls by Kei Kawashima-Ginsberg of Tufts University reveals that millennial support for a white woman in 2016 was 10 percentage points lower than their vote for a black man in 2008. Furthermore, the gender gap among young people was larger than in previous elections. While 63 percent of young women voted for Mrs. Clinton, only 47 percent of young men did so. ...