Call it the mommy penalty versus the daddy bonus.
A new government report finds that women with children under 18 earn less than women without minor children, while men with kids under 18 earn more than men who don't have younger kids.
Experts say it's further proof that, despite the many gains women have made in education and workplace equality in recent decades, mothers are still at risk of earning less because they also are parents.
"I think parenthood is like the new site of gender discrimination," said Michelle Budig, a sociology professor at the University of Massachusetts at Amherst.
Her research has shown that women generally make less money for each child they have, even after accounting for factors that would explain a salary reduction, such as taking time off for child-rearing and career choice.
Men, on the other hand, are more likely to see an increase in their earnings for being married and having a family after accounting for other external factors that would affect earnings, according to Budig's research.
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The Bureau of Labor Statistics report looked at the midpoint of weekly earnings for full-time workers. It found that in 2012, women with children under 18 had median weekly earnings of $680, versus $697 for women without children under 18.
Meanwhile, men with kids under 18 had median weekly earnings of $946, compared with $799 for men without minor children.
Overall, the BLS analysis showed that last year, women earned about 81 percent of what men earned in a usual week. That's a much narrower difference than when the government started tracking the data in 1979, but the improvements have become much slower and less steady in the past decade or so.
The gap between men's and women's earnings exists at all ages, but it is generally narrower when workers are young and widens more significantly at about 35, according to the latest BLS data.
That widening gap could be attributable to other factors, such as women's generally starting with lower salaries and less lucrative careers, and finding it harder to catch up over time, experts say. But many argue that parenthood probably plays a role.
(Read more: Office smackdown: Parents vs. childless workers )
Of course, some mothers choose less ambitious paths and thus lower-paying jobs because they have more child-rearing duties or want to be at home more.
But Francine Blau, an economics professor at Cornell University, said the problem comes when women who aren't making those trade-offs are treated as if they will be less productive because they have children.
"If employers believe on average that women with children would be less good workers, then they might discriminate against all women with children," she said.
Katherine Gallagher Robbins, a senior policy analyst with the National Women's Law Center, pointed out research showing that moms are often perceived-consciously or not-as less valuable workers. She noted a study in which participants were given two sets of rÃ©sumÃ©s, with one set including activities that would point to being a parent.
The rÃ©sumÃ©s that implied the woman was a mother generally got lower competency and commitment ratings and a lower recommended salary than those of a woman without children, while rÃ©sumÃ©s suggesting a man was a dad commanded a higher salary than those for a man without kids.
Budig's research shows that the effect on earnings is true for all parents. Looking more narrowly at white women only, however, she found that the mommy penalty is especially strong for low-wage women. They are more likely to have trouble balancing less flexible jobs with the often-unexpected demands of parenthood, like a sick kid, she said.
"Women who earn less pay more-they pay a higher proportional penalty for kids," Budig said.
Her research also has shown that married white moms pay a higher penalty than single moms after accounting for other factors, like age and the demographics of who gets married. But white women who are in the highest earnings bracket do not seem to be subject to the penalty.
In addition, her research found that the fatherhood bonus is especially true for well-educated white and Hispanic men.
"Men who conform to expectations of what makes a good man-being a highly educated, married father-[are] more valued as an employee," Budig said.
The reports come as working mothers are increasingly important to family finances. A Pew Research Center report released earlier this year found that women are either the sole or primary breadwinner in 40 percent of U.S. households with children under age 18, in many cases because they are single mothers. That compares with just 11 percent in 1960.
"It really is an economic concern not just for women but for their families in general," said Robbins at the law center.
(Read more: How the weak jobs recovery slammed men and women )
Blau, the economist, noted that families are changing in other ways and that men are much more involved in children's lives than they were a generation ago. The shift in attitudes about home life could eventually lead to changing attitudes about parents' position in the workplace.
As a practical matter, she said, more women are pursuing the type of educational specialty that is valuable to employers, such as a law degree or an MBAs. That could spur companies to start thinking more about keeping these highly educated moms happy, loyal-and well compensated.
"It's sort of incumbent ... upon employers to be concerned about these issues because they want to have the most productive workers," Blau said.
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