In corporate America, 88% of men think women have at least as many opportunities to advance as men.

This is the finding of a major new study—almost 30,000 employees across 118 companies—by LeanIn.org and McKinsey & Company.

Just 12% of men felt that women had fewer opportunities to advance in their organizations.

Yet when you look at the actual data, women’s odds of advancement are 15% lower than men’s.

It’s not because women are less capable: the evidence is strong that although men tend to be more confident leaders, on average women are more competent leaders.

And it’s not just a glass ceiling: women face bottlenecks in the middle and sticky floors. At every level, women are less likely to advance.

Why don’t men see it?

Hypothesis 1: Men are stupid.

There are more men than women with low intelligence. And for the past 20 years, 318 Darwin Awards have recognized people who removed themselves from the gene pool through “idiotic behaviors”—like the terrorist who mailed a letter bomb without enough postage, and when it was returned to sender, opened it.

It turns out that more than 88% of the Darwin winners were male. As the researchers write, “This finding is entirely consistent with male idiot theory… and supports the hypothesis that men are idiots and idiots do stupid things.”

But not all men are idiots. What accounts for the rest of the ignorance?

Hypothesis 2: Men are blind.

“Seeing is believing,” Ashleigh Brilliant once remarked: “I wouldn’t have seen it if I hadn’t believed it.”

Many intelligent men have a hard time stomaching the idea that the world of work still isn’t fair. Not long ago, I was one of them. Some men ignore gender inequality altogether. Others make excuses: “We have lots of opportunities for women. They just keep leaving to have kids.”

That’s not what the new study shows. In reality, women are less likely to leave their organizations than men—especially in leadership roles. Women in senior vice president roles are 20% less likely than men to leave, and women in the C-suite are nearly half as likely as men to leave. And women without children are less motivated to reach the top: mothers report 15% higher interest in becoming a top executive.

Progress toward gender equality in corporate America has slowed to a glacial pace. As Sheryl Sandberg wrote this week:

"At the current pace of progress, we are more than 100 years away from gender equality in the C-suite. If NASA launched a person into space today, she could soar past Mars, travel all the way to Pluto and return to Earth 10 times before women occupy half of C-suite offices. Yes, we’re that far away… We reached the moon in eight years of concerted effort—not 80. Let’s bring that same urgency to this mission.”

Five months ago Sheryl tragically lost her husband, Dave Goldberg, a strong advocate for women. He would have celebrated his 48th birthday this week, and I can think of no better way to honor his legacy than to mentor and sponsor women.

So men, repeat after me:

Women do not have the same opportunities to advance as men.

This is unacceptable.

I am going to take action to change it.

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Adam is a Wharton professor and the New York Times bestselling author of Give and Take. Sign up for his free newsletter on work and psychology at www.giveandtake.com.