David Letterman had Patricia Arquette on as a guest this week. She had recently won an Oscar, given a speech at the Oscars, and also has a new television show CSI:Cyber.

David Letterman congratulated her on all these things. But Arquette has not seen all positive response from what she said.

Patricia Arquette won the 2015 Oscar for Best Supporting Actress for her role in Boyhood.

After thanking her family and children, her heroes in her ecological efforts, and her film cast and crew, Patricia Arquette used her moment in the spotlight to draw attention to a cause near and dear to her heart.

“To every woman who gave birth, to every taxpayer and citizen of this nation, we have fought for everybody else’s equal rights. It’s our time to have wage equality once and for all and equal rights for women in the United States of America.”

The crowd roared in applause and approval. Meryl Streep jumped to her feet and enthused in response.

Backstage, Patricia Arquette added this:

“And it’s time for all the women in America and all the men that love women, and all the gay people, and all the people of color that we’ve all fought for to fight for us now.”

Her point: There is no law in America prohibiting women from being systematically paid less than men when performing the same jobs. The idea of a comprehensive Equal Rights Amendment has been floated many times over the decades, but has never gotten enough traction. An Equal Pay Act did pass in 1963 and was signed by JFK. President Barack Obama backed that with the Lily Ledbetter Fair Pay Act in 2009. But Patricia Arquette and other insist that something is still mucking up the works, and they have the numbers to prove it.

According to the American Association of University Women (AAUW), In 2013 women were paid 78 percent of what men were paid. That figure is sometimes explained away by claims that the numbers are skewed because women choose lower-paying jobs or work part time more than men do. But he AAUW is quick to note that this figure is among full-time, year-round workers.

They also point out that women are paid less than men, even in female-dominated occupations. And once a women reaches the age of 35, the gap between what she and a comparable man earn grows wider, from 90 percent to more like 75-80 percent.

Even comparing men and women fresh out of college with no children, women only earned 82 percent of what their male counterparts earn.

Other research shows that the pay gap is slowly narrowing, but so slowly that it would take until 2058 to see a true leveling of the field.

But why would an Oscar-wining celebrity like Patricia Arquette — a woman who no doubt has plenty of money — take up this battle?

In addition to talking more about this with David Letterman this week, Arquette spoke before a U.N forum recently, alongside Hillary Clinton and Melinda Gates, that launched the U.N Women project Planet 50-50 by 2030. There, Patricia Arquette dug deeper into why she said what she did at the Oscars.

“People have asked me why I’m doing this, and it’s true, today I’m blessed, having some material success, for which I’m extremely grateful. But I have other truths, too. If I were to tell you as a child, there were times where I lived below the poverty line, literally not having shoes to wear that fit me, that would also be true. If I told you that I was a single mother at 20, and lived with my baby in a converted garage, and that I would worry about my baby’s nutrition while nursing, because I could only afford to eat macaroni and cheese mixed with water for a week so I could afford diapers, that would also be true.”

“So let’s be honest: We have in place fair-pay laws that are not ensuring fair pay to women. The effect of the gender gap is most oppressive for women of color. In the United States, Latina women working full-time are taking home 56 cents to every dollar earned by her male, white co-workers. In California, which is the seventh-largest economy in the whole world, that number dropped last year to 44 cents on the dollar. African-American women earn just 64 cents. White American women may now average 78 cents on the dollar.

“Countless lesbian women and women in the transgender community also suffer wage penalties that hurt them and their families. And the women in our transgender community are suffering even more; most are not even able to get a job.”

“This is about supporting families, and getting women what they have already earned for their own hard work. Hold your lawmakers accountable … We are a movement, and we are making changes for our daughters. We matter.”

David Letterman asked Arquette if she had a single explanation for why women were still not getting equal pay.