David Buss, a professor of psychology at the University of Texas-Austin, elaborated on Geary’s point in an email, stressing the lower proportion of men than women getting college degrees:

Women have strong mate preferences such that they do not want to mate or marry men who are less educated, less intelligent, and less successful than they are.

And this, Buss said, “creates a surplus of men” at the low end who are not going to get married.

Millions of these less well educated men are not going to get the benefits of marriage:

Married men live longer, are less likely to become alcoholic, take drugs, commit suicide, etc.

In a phone interview a number of years ago, Richard Freeman, a Harvard economist, was prescient:

Men are really going to have to change their act or have big problems. I think of big guys from the cave days, guys who were good at lifting stuff and hunting and the things we got genetically selected out for. During the industrial revolution that wasn’t so bad, but it’s not going to be there anymore.

Asked to confirm his earlier views, Freeman wrote me that what he predicted

has occurred and continues, and perhaps is linked to the penchant for some male workers to be more favorable to right-wing populism than might have been the case.

David Deming, a professor of public policy at Harvard’s Kennedy School, suggests that things are not as simple for men as “changing their act.”

In a 2015 paper, “The Growing Importance of Social Skills in the Labor Market,” Deming writes:

High-paying, difficult-to-automate jobs increasingly require social skills. Nearly all job growth since 1980 has been in occupations that are relatively social skill-intensive. Jobs that require high levels of analytical and mathematical reasoning but low levels of social interaction have fared especially poorly.

What this means, according to Deming, is that

the economy-wide shift toward social skill-intensive occupations has occurred disproportionately among women rather than men. This is consistent with a large literature showing sex differences in social perceptiveness and the ability to work with others.

Studies of gender differences, according to Deming, show that

Females consistently score higher on tests of emotional and social intelligence. Sex differences in sociability and social perceptiveness have been shown to have biological origins, with differences appearing in infancy and higher levels of fetal testosterone associated with lower scores on tests of social intelligence.

In an email, Deming suggested two reasons that men may be reluctant to take jobs in the growing service sector. The first, he said, is that

if service sector and other “pink collar” jobs were higher-paying and more secure (perhaps unionized), they would attract more men.

The second reason, in Deming’s view, is that

many service sector jobs involve “serving” people of higher social status. I think women are more willing to do this — for cultural or genetic reasons, who knows.

From another perspective altogether, Allan Schore, a professor of psychiatry and biobehavioral sciences at the U.C.L.A. School of Medicine, explores the slower development among boys “in right-brain attachment functions.”