The #MeToo movement, which documents collective experiences of sexual harassment, has prompted an uncomfortable question: Why don’t other men speak up and stop sexual harassment?

Part of the answer is that men aren’t always able to recognize sexist behavior in others—and in themselves. In some research, both men and women exposed to acts of sexism notice and accurately describe the behavior, yet men are less likely to label it as sexist. Men are even less likely to label a behavior as sexism if it’s subtle or superficially benevolent, such as when a man refers to a woman as “nice, nurturing, or maternal,” thereby undermining her status as a competent, take-charge leader. Men also hold certain biases and stereotypes that may cause them to see a situation differently than women.

Both men and women exposed to acts of sexism notice and accurately describe the behavior. Yet men are less likely to label it as sexist.

For instance, take the pervasive stereotypes of men as breadwinners and women as homemakers. These ingrained stereotypes—men are assertive and aggressive, women are more passive and communal—tend to exacerbate status and power differences. Stereotypes might not only justify men’s sexist behaviors but also become internalized by women who become silent and self-blaming in the face of sexism and harassment.

Outside of their perceptions of others, men can also struggle to see their own behavior clearly, a finding that some say can be explained by biological and evolutionary theory. Evolutionary psychology, for instance, asserts that men have a stronger innate drive to find a mate. According to this theory, men can intend their actions to mean one thing without realizing that they come across differently (that is, as harassment) to women. As a case in point, men are more prone to the sexual overperception bias, which shows up when a man finds a woman attractive and thinks (falsely) that she feels the same way about him. The evidence-based message for guys in the workplace? Chances are, she isn’t that into you.

Yet another line of research shows that men react very differently to asymmetric power dynamics than women do, a tendency that can fuel sexist behavior. When men who are already prone to be predators (according to a “likelihood to sexually harass” scale) have power over a woman in the workplace, they may be vulnerable to a power–sex association, which makes them even more attracted to subordinate women.

The evidence-based message for guys in the workplace? Chances are, she isn’t that into you.

Although these social and biological explanations suggest why sexism occurs, they can’t and don’t excuse men from the ethical obligation to take responsibility for their actions and act less sexist.

One important antidote to male sexism and indeed the documented perception problem in the workplace is encouraging men to be allies of women—that is, to push for equal rights. In our research on cross-gender mentoring, we explored the best practices of many men who routinely step up and not only call out sexism but collaborate proactively with women to promote gender equity for the good of their organization.

Allies are essential to improving workplace culture: While men are more likely to overestimate their male peers’ perceptions and acceptance of sexism and often feel like it isn’t their place to confront sexist behavior, the reality is that men are more effective and incur fewer costs than women who take on workplace sexists.

Research demonstrates that men, compared to women, are seen as more legitimate and credible when they confront sexist behavior, in part because, as members of the out-group, they’re seen as acting in the absence of self-interest. And there are other effects: Research shows that women report more self-confidence, more self-esteem, and less self-stereotyping when men confront sexist behavior than when a woman calls out sexism. This same research also finds that women are more likely to report sexual harassment when male allies speak up. Ally confrontations empower others to challenge not just sexism but also other discriminatory behavior directed at minority groups.

Men are seen as more legitimate and credible when they confront sexist behavior, in part because, as members of the out-group, they’re seen as acting in the absence of self-interest.

So how do we develop, facilitate, and reinforce male ally behavior? First of all, ignore those like Vice President Mike Pence who advocate gender segregation in the workplace. The fact is that men need more interaction with women, not less. Having men work alongside women as equals reduces prejudice and discrimination. Cross-gender mentoring programs and peer mentoring programs can inspire more positive male-female interaction within organizations. So can encouraging men to participate in employee resource groups (ERGs) that were formed for women to share how their work experiences may differ from men’s. As the minorities in these groups, men are in the perfect spot to listen to women, build empathy, and check their assumptions.

In our book, Athena Rising, we interviewed Robert Lightfoot, acting NASA administrator, who told us about the (wrong) assumptions he made about the needs of the mothers that he mentored in the workplace—and about the female colleague who pointed this out. His solution to keep those gendered assumptions in check? Ask lots of questions. Like Lightfoot, effective male allies have a healthy dose of gender humility and are willing to admit mistakes and ask for feedback. Most importantly, as leaders in their organizations, they communicate the successes of male allies and advocates—helping other men view their workplace with different eyes.