NEW YORK (CNNMoney) - Joya Misra, a sociology professor at the University of Massachusetts, has a simple piece of advice for her female graduate students pursuing a career in academia.
"There's no reason for you in any way to indicate you're a mother," Misra tells them. "I want you to get the job."
The reason: There's a longstanding gap between the wages of mothers and those of childless women.
According to new research co-authored by Misra and two economists, that gap hasn't narrowed at all since the 1980s. And for some women, it's even increased.
The study found that when correcting for education, occupation and work experience, the pay gap for mothers with one child rose from 9% in the period between 1986 and 1995 to 15% between 2006 and 2014. For mothers with two kids, the gap held steady at 13% and stayed at 20% for mothers with three or more children.
The paper draws upon the University of Michigan's Panel Study of Income Dynamics, which has tracked about 18,000 individuals from 5,000 families since it began in 1968, making it an unusually powerful tool to measure how wages respond to demographics and life events.
The "motherhood penalty," as it's been called, helps to explain why women overall make 81 cents on every dollar a man makes. Conversely, research has shown that having children actually raises wages for men, even when correcting for the number of hours they work.
Why hasn't the discrepancy eased, even as the share of mothers with young kids working has risen from 47% in 1975 to 70% in 2015?
The researchers point to a lack of progress on family-friendly policies in the United States, such as paid parental leave and subsidized childcare. Other countries, including Sweden, have narrowed their gender pay gaps after instituting such laws.
"It's really, really, really clear," says Misra, who has studied the policies different countries use to support mothers. "Universal subsidized childcare has the most important effect on reducing the motherhood penalty."
It's an issue confronting human resource managers, too. When it comes to handing out raises or promotions, they must decide how to treat the time off that women take after having a child.
"We're absolutely convinced that a significant part of that is women having to make choices in their childbearing years," says Society for Human Resources Management CEO Johnny Taylor, on the motherhood penalty. Mandating that employers provide paid parental leave would help, he says, but it won't stop employers from treating women differently for taking the time they need off after giving birth.
He offered the example of a law firm. "You're a female associate," Taylor says. "Should you be considered for partnership at the end of your seven years, when you took nine months off? We're trying to solve for that."
Of course, punishing anyone for having family responsibilities is illegal. Lawsuits charging discrimination on that basis have skyrocketed in recent years, according to a 2016 report by the Center for Worklife Law at Hastings College of the Law.
But the total number of cases remains small, and Misra says that the federal Equal Employment Opportunity Commission doesn't have enough money to proactively police the issue.
"If they had many more investigators, workplaces would tighten up more than they have," Misra says. "What they primarily have learned is what to say and not say in order to be within the law, rather than how to actually not discriminate."