On Wednesday, former Microsoft employee Katie Moussouris filed a class-action complaint against the company in Seattle federal court. Moussouris alleges that female engineers at Microsoft have been systematically undermined, underpaid, and under-promoted. The suit cites instances of Moussouris receiving worse peer evaluations than her managers felt she warranted; male coworkers she claims were less qualified receiving promotions over her; and her receiving lower pay than male counterparts throughout her time at the company.

The heart of the suit is the claim that Microsoft’s employee evaluation methods, primarily the controversial stack-ranking system Microsoft used until late 2013, are sexist. In that system, groups of employees ranked their peers based on performance, and raises and bonuses were distributed accordingly. Microsoft declined to explain its current employee evaluation process.

Upon information and belief, female technical employees tended to receive lower scores than their male peers, despite having had equal or better performance during the same performance period,” the suit reads.

It’s challenging to prove beyond reasonable doubt that an employee was undervalued, but there is one area of Moussouris’ suit that does stand out in this regard. The suit says that when Moussouris was on maternity leave in 2012, she was passed up for a promotion “for which she was eligible and eminently well-qualified.”

“In the year prior,” the suit reads, “Plaintiff had been responsible for an industry-leading initiative that resulted in the highest-rated news announcement for the Microsoft Trustworthy Computing Group. However, Microsoft instead selected Plaintiff’s male colleague, who was less qualified, to be her manager.”

The rest of the suit details instances of implicit bias, such as managers being overly critical of a woman’s attitude. But the criticisms of stack ranking, which can be extrapolated to many forms of comparative evaluation, coupled with treatment of women on maternity leave are the most salient aspects of the complaint.

Nicole Bates, a program manager at Intellectual Ventures, worked at Microsoft from 2008 to 2011. She gave birth to her son in 2013, and her employer strongly influenced one of the major decisions a parent must make. “Had I worked for Microsoft (when she gave birth), I probably would have quit,” she said.

“At Microsoft, for my one group that I worked with, if you weren’t in your seat by 9 o’clock, you’re totally worthless that day. If you left at 4:30, clearly you are a slacker.”

This type of culture, as we’ll detail in the October issue of 425 Business, disproportionately affects women. Studies have shown that cultural and institutional norms nudge women toward the primary child-rearing role. Take Microsoft’s new parental leave policy, which goes into effect in November, that offers women 20 weeks of paid parental leave and men eight. If a family wants to maximize the amount of time a mother or father spends at home with the newborn baby but doesn’t want to take unpaid leave, it’s clear the mother staying home is the more practical choice.

Moussouris’ suit alleges that, once that mother returns to work, she might find herself discredited by her peers, and thus paid less once raises, promotions, and bonuses are doled out.

According to employees, this was commonplace in the stack-ranking era. When women who had been on maternity leave were being measured against their male coworkers, the maternity absence was often brought up.

“The conversation we have when we go into stack ranking is, ‘Oh, well she was out for three months,'” said Robert O’Twomney, a program manager at Microsoft. “I will absolutely stop that conversation. The three months are taken off the books. … It always pops up, at every stack ranking at every company I’ve ever been at.”

That these conversations come up during evaluations doesn’t inherently put Microsoft at fault. Just as O’Twomney decides to stop those conversations, it’s the decision of individuals on the stack-ranking panel, not the company as a whole, to broach the topic. What could be damning, at least in the court of public opinion, is Moussouris’ claim that after she filed an internal complaint, Microsoft took “adverse actions against Plaintiff with the purpose of retaliating,” though the suit doesn’t divulge what those actions were.

Microsoft, for its part, said in a statement that it reviewed and could not substantiate Moussouris’ claims. And substantiating that Microsoft’s employee review system is biased against women would be tricky — is a company responsible for sexist views that its employees might hold? Microsoft can implement fair or unfair evaluation strategies; what it can’t do is control the way its employees think. The question at hand, then, is whether the former has significantly influenced the latter, thus leading to unfair treatment of women in the company.

This post has been updated.