Many women without children fear that the prospect of motherhood might be treated as a risk by employers: that they could be passed over for a job or promotion on the basis that they might have children in the future.

Now, a large study on hiring bias has suggested the fear has a foundation, at least for women seeking to work part-time. It discovered that women in their 30s who were married and childless were less likely to be called back by employers seeking part-time staff than almost any other female group.

For the study, economists sent out applications to over 9,000 full-time and part-time secretarial and accountancy jobs advertised in major cities in Germany, Austria, and Switzerland. In these countries, it is standard to include information about marital status and children on resumés, part of the reason they were selected for the study.

All the invented candidates for the roles were 30, but their characteristics varied. Some were male, some female, and they had different family structures: single and childless, married and childless, married with young kids, and married with older kids, information which was included on the majority of the resumes. As a control, they also sent out some applications that had no family information. The researchers then measured the number of callbacks each of their fictitious applicants received.

The male groups were called back less than the female groups. This was in line with the researchers’ expectations, since they had deliberately chosen female-dominated fields in order to test whether it was family structure—and not gender—that had an effect on callback rate. Among women applying to part-time jobs, those who were married and had older children were the most likely to be called back, while married but childless women were the least likely to be called back. A gap of 14 percentage points separated the two groups.

“Our conjecture is that employers consider childless, but married women, at particular ‘risk’ of becoming pregnant,” the researchers wrote. When it came to full-time jobs, the significant disparity disappeared. The researchers speculated that employers receiving applications for full-time positions judged that women must already have childcare in place, while those seeking part-time work signaled a desire for flexibility.

“We interpret these findings as presence of substantial hiring discrimination based on realized and expected fertility for part-time jobs—a possibly surprising result, since these jobs are typically meant to be particularly family-friendly,” the economists noted. The research was published this month by the IZA Institute of Labor Economics based in Bonn, Germany.

The study puts some numbers to the fear 30-something women have that they will be perceived as a potentially risky hire, no matter what their intentions are with regard to starting a family. Biology dictates that women bear children, but gender norms suggest that it is women who will take on the bulk of childcare responsibilities, the researchers noted.

Among women who do have jobs, a significant “motherhood penalty” already exists globally, with women who have children suffering a large fall in income immediately, and experiencing an ongoing salary impact that lasts a lifetime. Part of this is explained by women moving to part-time work after becoming mothers; part is down to women being promoted less, and their work simply being less well-remunerated than that of men.

The IZA research is striking because it highlights a specific injustice: Women are having to contend with the motherhood penalty before they even decide whether to have kids or not; and it’s just as bad for young, married women who never plan to become mothers as for those who do.