– Anna Dapelo-Garcia has managed employees for about two decades, but she's never had women ask for a pay increase or negotiate their salaries in the way that men do.

"I've had males ask me for more money, and it amazes me how confident they are in doing it," said Dapelo-Garcia, 54, an administrative director for patient access services at Stanford Health Care in Stanford, Calif.

Women on average are paid 20 percent less than men in the United States, according to census data compiled by LeanIn.Org and the National Partnership for Women & Families. Black women are paid 37 percent less than men while Hispanic women make 46 percent less, data show. In California, women are paid 14 percent less than men, which is lower than the national average.

The reasons for the gender pay gap are multifaceted. Research shows, though, that discrimination and unconscious bias along with differences in experience and education are some factors, according to a fact sheet by the National Partnership for Women & Families.

"Equal pay is essential to the goal of gender equality," said Sheryl Sandberg, Facebook's chief operating officer and founder of LeanIn.Org, in a statement. "This issue speaks to how we value women's labor, knowledge, time, training and so much more. In short, it's about women's worth. There's nothing more fundamental than that."

LeanIn.Org is a women's empowerment initiative started by the Sheryl Sandberg & Dave Goldberg Family Foundation. It sponsored a national campaign called 20PercentCounts, aims to raise awareness about the pay gap that exists between men and women and encourage others to take action.

Rachel Thomas, president and co-founder of LeanIn.Org, said in a statement that the campaign is more than just about public awareness.

"It's about all of us taking action. Elected officials have a role to play in passing and enforcing the right laws. Businesses need to audit pay by gender and race so they can see and address pay gaps — and work to make sure performance reviews and promotions are fair. And we all need to celebrate, and not penalize women, when they advocate for themselves and ask for more," she said.

Closing the pay gap would lift 3.1 million working women and their families out of poverty and add $513 billion in new income each year to the U.S. economy, according to research commissioned by the Institute for Women's Policy Research.

Growing up in San Jose and the first in her family to go to college, Dapelo-Garcia recalled that her parents never talked to her about salary negotiation.

"Especially when you don't really have power in the form of education or influence, you don't want to rock the boat," she said.

Dapelo-Garcia started and leads a LeanIn Circle for Latinas, which has more than 400 members across the country.

By raising more awareness, Dapelo-Garcia hopes that the gender wage gap will become a thing of the past. "In order for us to close this wage gap, we're going to have to be loud about it," she said. "Women need to be paid fairly for doing the same work."