Female college students are 1.5 times as likely as their male counterparts to leave science, technology, engineering, and mathematics (STEM) after taking the first course in the calculus series, new research finds. The study, published last week in PLOS ONE, supports what many educators have observed and earlier studies have documented: A lack of confidence in mathematical ability, not mathematical capability itself, is a major factor in dissuading female students from pursuing STEM.

The researchers followed 2266 undergraduate students at 129 2- and 4-year colleges and universities who were enrolled in Calculus I, the first course in a calculus series that is often a prerequisite for studying STEM disciplines in the United States. Overall, students were more likely to continue with calculus if they were planning for careers in engineering, had good instructors, or had previously scored well on math SAT and ACT standardized tests, the researchers found. However, when comparing students with the same background, experience, and plans, female students were on average 50% more likely than males to stop studying calculus, “effectively choosing to exit the STEM pipeline,” the authors write. It’s very natural for students to start college and realize that they are interested in pursuing a discipline other what they initially expected, says study lead author Jessica Ellis, an assistant professor of mathematics at Colorado State University, Fort Collins, “but there’s no reason that it should be happening at different rates in different populations.”

Female students cited one reason for leaving more often than their male classmates did: believing that they did not understand the concepts of Calculus I well enough to go on to the next course. In fact, 35% of surveyed female students who had initially intended to major in STEM but decided against taking Calculus II selected this reason, as compared with just 14% of male students. (Other reasons for leaving—changing majors, too much time commitment, poor class experience, and an inadequate grade in Calculus I—were cited approximately equally by both genders.)

The researchers found that male and female students lost confidence equally over the course of the term, but female students entered the course with a lower confidence level. Therefore, the decision to stop pursuing calculus—and, by extension, to leave STEM—appeared to be influenced significantly by math confidence when entering college, the authors conclude. “The sad thing to me [is that] it wasn’t that women were losing confidence at higher rates than men,” Ellis says. “It was that they were coming in with lower confidence.”

The authors note that addressing this confidence mismatch could help boost the representation of women in STEM. If women stayed in STEM after Calculus I at rates similar to men, the number of women entering the STEM workforce would increase by 75%, the authors project, and women would make up 37% of the entering STEM workforce, a step up from the 25% they currently compose.

The high attrition rate for female students due to low confidence is not surprising to those involved in STEM education. Few role models or peers and preconceived conceptions about the relationship between gender and STEM ability don’t help. “If everyone expects you to do worse and people don’t look like you, it’s discouraging,” says Shulamit Kahn, a professor in the Questrom School of Business at Boston University who studies gender gaps in the STEM workforce. And the view by many science and engineering departments that calculus can be used to “weed out” weaker students does not encourage confidence in general. “Our culture says that it’s OK in STEM that not everyone does well,” says Tobin Smith, vice president for policy at the Association of American Universities (AAU) and co-principal investigator of AAU’s Undergraduate STEM Education Initiative. Introductory classes in particular, which historically have not been the primary focus of departments, “are always a problem and have not been taught well,” he says.

Many educators believe that the solution is active learning, which encourages student participation through approaches including smaller classes, group work, completing problem sets in class, and receiving immediate feedback from instructors. The general consensus is that active learning classes improve student performance and confidence.

The study found that this type of “student-centered” teaching did not affect the likelihood that students—male or female—would move on to Calculus II, but there are a few caveats. At the time the survey was administered, in the fall of 2010, most Calculus I classes were traditional lecture courses, so there may not have been enough active learning classes at the time to detect their influence, the authors suggest. It is also possible that students may not have liked active learning instruction because it was different from what they were used to and what they expected from a college-level class, the authors write. Active learning classes take time to develop, notes Mark Graham of the Center for Teaching and Learning at Yale University, but “when it works and it is implemented well, I think it addresses a lot of the issues this paper brings up,” he says.

Correction: This article has been corrected to clarify that female students are 1.5 times as likely to stop studying calculus, not 1.5 times more likely.