Researchers already know that women are more likely to quit studying science, technology, engineering, and math (STEM) than men are. There are a lot of reasons for this, including faculty bias that favors male students.

But new research suggests male students are also biased toward their male peers. This can undermine women's confidence and make them feel less included in their field, especially at pivotal decision-making moments in their career.

Male biology students showed 19 times more gender bias than women

Researchers from the University of Washington asked 1,700 students enrolled in undergraduate biology classes to identify who in the class was particularly strong at the material and would do particularly well in the class.

The results were startling. Male students consistently identified other male students as more knowledgeable, even over better-performing female students. And the bias was huge. Men gave their male peers a "boost" equivalent to three-quarters of a GPA point on a 4-point scale.

In other words, a woman with a 3.75 GPA and a man with a 3.00 GPA would have roughly equal chances of being nominated as smartest in the class by a male student. This was the case even controlling for factors like male students being rated more outspoken overall, and male students having higher grades overall.

Women, on the other hand, didn't show much gender bias at all in whom they chose. They showed only a slight bias toward other female students, equivalent to a .04 GPA boost. So researchers estimated that men's gender bias was 19 times greater than women's.

The study shows that millennials still have anti-woman biases in STEM

Some conventional wisdom holds that as younger generations mature, bias against women in STEM will improve. Sure, older faculty might have gender bias, the theory goes, but that will improve with time. But this study suggests that view is overly optimistic.

"Because these are millennials showing this pattern, it means the age-old problem of gender bias may not go away simply because we have a new generation in charge," lead author Dan Grunspan said in a statement.

And the anti-woman bias could be even worse in other STEM fields, researchers noted. They chose a biology class partly because women and men enroll in roughly equal numbers in undergraduate biology classes. In fields where the gender disparity is worse from the get-go, the gender bias is also likely to be worse.

Undergraduate biology classes were also chosen, researchers said, because introductory STEM classes are a key "transition point" that could make or break a woman's choice to stay in a particular field.

During such transition points, like the period leading into college or graduate school, people tend to be especially sensitive to social cues and feedback about whether they're cut out for a certain subject area. And the implicit feedback women in STEM get from their male peers doesn't seem to help.

What could help, researchers suggest, is more female instructors, woman-only study groups, randomized lists to call on students in class, or small-group discussions. But battling systemic bias is tough no matter what.

"As science instructors at the college level, you can only affect so much," said co-lead author Sarah Eddy. "There's been at least 18 years of socialization. You do what you can to interrupt that."