Onlys face more challenges in the workplace than other women, the survey found. Half of these women said they need to provide more evidence of their competence than others do. Onlys are twice as likely as other women surveyed to be mistaken for someone junior. These women are also almost twice as likely to be subjected to demeaning comments and twice as likely to report having experienced sexual harassment in their careers.

“This was my entire career, basically,” said Kristen Fanarakis, who spent 15 years working in finance. She was a part of many all-male teams and didn't have a female friend at work until she was in her 30s, she said.

Although Fanarakis had many supportive male colleagues and mentors over the years, she said, other men treated her with disrespect. One boss told her she could “handle the nappies,” she recalled. Another held her to impossible standards, giving her poor performance reviews even though she brought in new business and met all of her goals, she said.

These experiences are even worse for women of color. Almost half said they're often the only person of their race at work. These women are more likely to feel excluded, scrutinized and closely watched, the survey found. Maura Cheeks, an MBA student, has written about having her identity mistaken for another black woman in the office and having to explain her credentials to colleagues.

More than 90 percent of the companies surveyed said diversity and inclusion is a top priority, but for the fourth year in a row, Lean In and McKinsey found that corporate America has made almost no progress in increasing women's representation in the workplace. Women make up 48 percent of entry-level employees but only 22 percent of the C-suite, as companies fail to promote women, the study found.

Even those who do make it past the early stages of their careers aren't likely to stay. Onlys are more ambitious than other women, the Lean In/McKinsey study found: Almost half said they want the top job, and almost 80 percent said they want to be promoted. But they're less likely to stay at their own company; more than a third of Onlys said they're thinking about leaving their jobs in the next two years.

“You have a group of women who are put in very isolating and scrutinized positions,” said Rachel Thomas, the co-founder and president of Lean In. “You would hypothesize the reason they are leaving is because they are having an experience that is markedly worse than other women.”

Fanarakis, for one, eventually left her finance career behind. Being an Only “takes a physical and emotional toll,” she said. She went to business school, wrote a book, and in 2017 launched a women's workwear startup.

Thomas suspects that many companies check the diversity box by hiring just one or two women. It's a strategy that hurts more than it helps, she said.

At her first job out of college, Molly Oswaks was one of two women on staff at the tech news site Gizmodo. She said her colleagues posted pornographic content in the company group chat, adding that readers would harass her in the comments section of her stories, saying she'd slept her way to the job. “There was no respect for the fact that women were there,” she said. “It was just like, I was the girl that they plucked to have a girl on staff.” (Gizmodo did not immediately respond to a request for comment.)

The benefits of diversity don't kick in with tokenism. Studies have found that if women make up 20 percent of a group, they account for only 10 percent of the conversation. Women need to constitute a supermajority to make up 50 percent of talk time in a group.

“These companies want a diversity of ideas,” Thomas said, but hiring the bare minimum number of women produces “a diluted form of diversity.”

Read more: 

• How do women and men experience work differently?
• Women and men still have very different career expectations
• Young women aren't closing the Great American gender-pay gap

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