To “Man up” means to “demonstrate toughness or courage when faced with a difficult situation.” That’s precisely what men need to do. Face up to a changing world and redefine what it is to be a man. In his recent book “Of Boys and Men,” Brookings Institution scholar Richard V. Reeves details how American men face a crisis of enormous proportions. The evidence is abundant.

Three of every four who die of suicide or drug overdoses, “deaths of despair,” are men. Men face a widening gap in higher education and a diminishing portion of the labor market. Women now constitute nearly 60 percent of university students and, more telling, graduate at 50 percent, while men complete their college education at a disappointing 33 percent. At every level, high school, associate degree, four-year and post-graduate level, men graduate at a lower percentage than women.

Women, freed from former societal constraints, show greater ambition, motivation and initiative. Opportunities are more gender-neutral, and women are stepping up to the full range of positions while many men are not. They appear stuck in antiquated stereotypes.

Men are taught to withhold their emotions, stoically tolerate pain, and be tough on the outside. It’s been a bar that measures manhood. It’s an outdated measure in a society where women have advanced into new fields while men seem adrift, unable to see the full range of possibilities.

“Make America great again,” attractive to a wide swath of American men, can be seen as a wish to return to the male dominance of the past. The expansion and growth of women’s roles have left many men uneasy. Legislation against transgender people, sex-neutral bathrooms in our public schools, violence against gays, and the growth of militia groups reflect men unhappy with changing gender definitions.

Historically, masculinity has been built on three pillars: provide, protect and procreate. When families are smaller, the conventional picture of a male is a thing of the past. When women come to compete or outproduce men economically, the traditional definition of manhood is obsolete. When male protection is rarely needed, the definition of maleness is outmoded.

Jobs dominated by men just two or three decades ago are now open to both sexes. Women entering virtually every occupation during the last 50 years support that fact. Veterinarians, pharmacists, psychologists and physicians are among the most high-profile careers, and the tip of the iceberg of women’s infusion into previously male-dominated professions.

Meanwhile, elementary school teachers, airline stewards and nurses are in demand, with solid-paying careers that many men refuse to enter because of masculine stereotypes. It’s not because of a lack of male capability but because men fear the stigma of femininity.

We have endured centuries prohibiting women from fulfilling their talents because of prejudice against their gender. Now men limit their opportunities because of self-inflicted attitudes — beliefs that it’s not manly to show empathy, caring and emotional vulnerability. Such characteristics are plentiful among men, but we need to accept them as qualities of strong, wholly masculine men.

“Man up” today still means demonstrating toughness or courage when faced with a difficult situation. But today’s difficult situation is to abandon outdated stereotypes and for men to embrace and employ all their healthy qualities. It is time for men to break the shackles of a narrow definition of masculinity that confines their emotions and even limits their longevity. Women are living, on average, about six years longer than men.

In less than 100 years, women in most Western societies have expanded their nations’ culture, science and leadership, with talents suppressed for centuries. We can’t put women’s gains back in the bottle. Nor should we want to. We’re the better for it.

Wake up, men. It’s time to recognize that expanding the definition of manliness to include empathy, caring and emotional vulnerability will make us healthier and ultimately stronger.