A federal appeals court ruled Monday that employers cannot justify paying a woman less than a man doing similar work because of her salary history -- a move advocates say will help close the wage gap between the sexes.

The U.S. Court of Appeals for the 9th Circuit sided Monday with the California math consultant at the center of Rizo vs. Fresno County of Education, who argued that considering prior compensation when setting a worker's pay perpetuates gender disparities and defies the spirit of the Equal Pay Act.

"The Equal Pay Act stands for a principle as simple as it is just: men and women should receive equal pay for equal work regardless of sex," wrote Judge Stephen Reinhardt in the opinion . "The question before us is also simple: can an employer justify a wage differential between male and female employees by relying on prior salary? Based on the text, history, and purpose of the Equal Pay Act, the answer is clear: No."

Aileen Rizo, who trained educators on how to better teach math, sued her employer of three years, the Fresno County of Education, in 2012 after learning her male colleagues made significantly more money than her despite having less experience.

In court, Rizo's employer admitted her salary was lower and argued the discrepancy stemmed from her prior salary - which, it asserted, had nothing to do with her gender.

In the opinion, Reinhardt disagreed with this logic. (He died last week at age 87 offering Trump an opportunity to appoint a new judge.)

"Before this decision, our law was unclear whether an employer could consider prior salary, either alone or in combination with other factors, when setting its employees' salaries," he wrote. "We now hold that prior salary alone or in combination with other factors cannot justify a wage differential."

In the United States, women earn an average of 82 cents for every dollar paid to men, according to the latest Pew Research Center analysis of median hourly earnings.

This is a leap from the 1980 figure (60.2 cents for every dollar), but the chasm hasn't narrowed much over the last 15 years, and it tends to be worse for women of color. Black women earn about 63 percent of what white men make, and the share is 67 percent for Hispanic women.

Economists say the gender wage gap boils down to career choice, breaks in work for maternity leave and childcare -- mothers in the U.S. still bear a disproportionate amount of the child-rearing burden -- and insidious forms of discrimination , such as assumptions that mothers won't be as dedicated to their jobs. (One 2015 study found women are judged more harshly for leaving work early than men.)

Ariane Hegewisch, a labor economist at the Institute for Women's Policy Research, said women, on average, are still paid less than their male counterparts in most industries. Companies that determine a worker's value based on prior pay, she said, exacerbate the problem.

"They import discrimination," Hegewisch said. "If you start at a lower salary, the gap in actual money will get wider and wider every time you get the same percentage increase as your higher-paid colleague."

Since Rizo sued her employer, Massachusetts and California have passed laws that block managers from requesting an applicant's prior salary.

Fatima Goss Graves, chief executive officer of the National Women's Law Center, said the federal court's ruling will make it easier for women to attain fair wages.

"We know one of the challenges women face is they carry low pay from job to job," she said. "They are losing because there is this huge loophole in our equal pay laws. This tightens it up."

-- The Washington Post