If you heed the wisdom of career experts on the Internet, talking about your kids during a job interview is a bad idea. It gives employers a reason to discriminate, especially against mothers who are presumed to have less than unlimited time to devote to their jobs (unlike, say, everyone else). In fact, to avoid breaking antidiscrimination laws, employers aren't supposed to ask perspective employees questions "involving marital status, number and/or ages of children or dependents, or names of spouses or children of the applicant," according to guidelines from the Equal Employment Opportunity Commission.

But there's new evidence that keeping the kids out of the job interview is harming some of the most vulnerable women in the U.S. workforce. 

Women who "opt-out"—taking an extended period of time away from careers to raise kids—already have a hard time reentering the labor market. Around a third of "highly qualified" women leave their jobs to spend time at home with their children, according to an oft-cited 2009 study from the Center for Talent Innovation. Almost 90 percent of those women wanted to go back to work, this study found, but only 40 percent got full-time jobs. The women who do make it back into a job take a financial hit, with compensation dropping about 30 percent after just two or three years away from work, according to the Institute for Women's Policy Research.

The unspoken rule of not speaking about your family might be prompting employers to take a dimmer view of women who have opted out. Researchers at Vanderbilt University asked more than 3,000 people to assume the role of employer and judge résumés of hypothetical female job applicants. All the résumés displayed equal qualifications, and all applicants had taken a chunk of time away from work. Some applicants explained the reason for the gap, with reference to raising children, and others offered no explanation at all.

Being candid about a child-rearing interval proved far more successful than silence, with employers becoming 30 percent to 40 percent more likely to hire a woman who offered an explanation for taking a break from work. "Any explanation is better than no explanation at all," said Jennifer Shinall, an assistant professor of law at Vanderbilt who worked on the study. "People prefer known risk, knowing why a woman left the labor market."

Awareness of a gap in your work history gives employers a reason to speculate—and discriminate. "The problem with the EEOC guidance is that it creates an atmosphere of ambiguity with regard to a person's care-taking responsibilities," Shinall said. "When the fact of the matter is that's a reality of every day life." Nearly three-quarters of women with children between the ages of 6 and 17 participated in the U.S. labor force as of 2013, according to the Bureau of Labor Statistics.

Of course, disclosure carries risk of its own. Telling a potential employer that you spent years caring for children might "greatly exacerbate that discrimination," Ofer Sharone, a sociologist at the University of Massachusetts, told the New York Times. Women are often discriminated against for having kids. The so-called motherhood penalty has been found to decrease a woman's salary 4 percent for every child she has. Talking about the kids at home during an interview would seem to invite similar bias, whether overt or not.

That sort discrimination will still show up at some point unless the topic of children stops being treated as a taboo. "Everyone needs to be talking about this," argues Shinall. "That's when we're going to see any sort of long-run change."