Until now, such discrimination continued unquestioned because of widely-held stereotypes about how women will or should act as mothers. Recent research into “cognitive bias”— or unacknowledged sets of stereotypes— reveals a number of stereotypes about mothers that can adversely impact her perceived job performance or qualifications for a job.

One of the most prevalent stereotypes is an association between motherhood and a lack of competence. “We find that people actually have underlying stereotypes in which they think of mothers as very nice people, but they don’t think of them as competent people,” says Professor Faye Crosby of the Psychology Department at the University of California, Santa Cruz, and member of the Cognitive Bias Working Group.

According to research conducted by members of the Cognitive Bias Working Group and published in the Journal of Social Issues, the link between incompetence and mothers is so strong that while subjects rate businesswomen as highly competent as businessmen and millionaires, the subjects rank “housewives” as minimally competent as the “elderly,” “blind” and “retarded” (the researchers used these stigmatized terms intentionally). Another study in the same journal shows that when men become fathers, they are held to lower standards of time commitment and competence, but mothers are held to higher standards of time commitment and must prove their competence in the workplace repeatedly.

Another common stereotype is that mothers are assumed to be less dependable than other workers. Stanley says of her former employer, “I think their actions, body language [and] tone of voice all suggested that I was no longer loyal to the company, that my mind would now be in another place and not on my work.” Such stereotypes about mothers’ commitment to work can result in women being passed over for promotions or receiving lower pay or lower quality assignments. Williams recalls the words of one Boston lawyer who was given the work of a paralegal after her maternity leave, “I had a baby, not a lobotomy.”

Fathers and people with aging relatives also face penalties for taking time to care for their families. For men the initial stereotyping is a little different. At first, men are not questioned on their competence and are perceived to be better managers because they are now “warm” or more personable as a result of being fathers. But things change if fathers want to do more caregiving work than is commonly expected of them.

In her book Unbending Gender, Williams discusses the employer’s concept of an “ideal worker”— a person who is available for long hours to work, largely because someone else attends to the caregiving. According to Williams, a father who seeks more time for caregiving may be “flunking the ideal worker test. He may be seen as a less effective man and even a less effective father, because fatherhood is very much intertwined with being a good provider.” The stereotype here is that fathers do not provide care. For example, in the caregiver discrimination case of Knussman v. Maryland, Knussman’s supervisor told him that his wife would have to be “in a coma or dead” for a man to qualify as the primary caregiver, and then denied him leave to care for his wife and newborn daughter.

“The Maternal Wall basically affects anybody who plays the role traditionally played by moms. If a man … wants to take several months parental leave or work a flexible schedule, he may experience more acute gender bias than the mother,” says Williams. “One of the reasons it’s so hard for moms to get dads to play an equal role [at home] is because of the fierce hostility [fathers] face in the workplace.”

Williams hypothesizes that the one of the reasons that some mothers leave the workforce and “opt out” has to do with the constraints on fathers. If fathers experience hostility for doing more caregiving work, then mothers are forced to pick up the slack. Says Williams, “[Women’s] partners feel that there is no way that they could back out without just triggering acute stigma.” In this way, the couple is forced into the ideal worker norm.

New research recently published by Hunter College sociologist Pamela Stone looks deeper into professional mothers’ decisions to leave the workforce and finds that approximately 90% of the subjects “expressed a moderate to high degree of ambivalence” about leaving their jobs. The study finds that it is the workplace factors that carry the biggest weight in their decisions, and then the flexibility of their partners.

“Many women name specific workplace conditions that led them to quit,” comments Williams regarding this research. “Sometimes that was the unavailability of part-time work, sometimes it was the stigma associated with part-time [work], sometimes it was the hostility towards women once they had children.” When such work conditions are combined with a partner who is inflexible in his work as well, then work becomes an all or nothing proposition. “That tends to mean that one partner, almost invariably the man, will stay on the 50 to 60 hour a week fast track, at which point his partner has very little option but to take very poor quality part-time work or leave the workforce,” notes Williams.

Such research supports Williams’ hypothesis that professional women are “opting out” of the workforce because of Maternal Wall discrimination. “One of the reasons that moms end up deciding that the only feasible solution is to sequence [or interrupt their careers] is because they themselves have met Maternal Wall bias and are sick of it,” Williams suggests. For Stanley, the former human resources manager, the hostility she experienced at work during her pregnancy certainly contributed to her decision to stay at home. “The work itself wasn’t so bad, but the environment was more than I wanted to handle. I couldn’t imagine coming home to my child in the moods this type of treatment brought on,” she says.

In the face of discrimination and hostile work environments, some mothers leave the workforce. Clarke, who worked in finance, describes the vicious cycle she saw created: “A lot of women do leave, and so that’s the assumption that is what every woman is going to do…so you’re passed over for a promotion or there’s a pay differential.”

The blame, though, lies less with individual women’s choices and more with our lack of understanding and acknowledgment that gender discrimination is a legitimate source of distress for employed mothers, stay-at-home mothers, and fathers. Many women, upon becoming mothers, do want cut back hours, not travel, or spend more time with their children. But that does not constitute a rationale for assuming that all mothers will or want to do these things. “Employers will try to say that they are trying to prevent problems down the road…but it’s still illegal,” says Camacho, who experienced discrimination during her job interview.