STEM education — majors in science, technology, engineering, and math — can be a path to business success for women. Almost all of the women on Fortune’s Most Powerful Women list — including those not at tech firms — had majored in STEM studies. PepsiCo CEO Indra Nooyi, for instance, had a triple-major bachelor’s degree in physics, chemistry, and math. Mary Barra, chief executive of GM, was an electrical engineering major.

In some STEM fields, women have made strides. They are 39% of chemists and material scientists and 53% of professionals in the biological sciences. And yet, a new American Association of University Women review of more than 380 studies from academic journals, corporations, and government sources shows a big employment gap in computing and engineering. In some fields, the gap is actually widening over the years — not shrinking.

In 2013, only 26% of computing professionals were female — down considerably from 35% in 1990 and virtually the same as in 1960. While the percentage of women in engineering has risen since 1990, the progress is modest –rising from 9% in 1990 to 12% in 2013.

“[Engineering and computing] have 80% of the STEM jobs available and the best-paying jobs,” Christianne Corbett, an AAUW senior researcher, told Fortune. “And they typically only require a bachelor’s level degree of education for an entry-level position [rather than a graduate degree for many sciences]. They offer a greater return on educational investment.”

What’s driving these numbers? The report suggests the following factors are at play:

1. Even before college, there is gender bias.

In engineering, women fight “historic ties to male-dominated apprenticeships and powerful cultural norms [that] marked engineering as ‘male,'” according to the report. Higher early exposure to engineering and computing among boys in school creates “more positive attitudes toward and interest in STEM subjects” then girls ultimately exhibit. The early exposure is crucial, as interest in STEM fields in general during high school is associated with the ultimate pursuit of an engineering or computing education or career.

By the time they reach college, one out of five young men plan on majoring in engineering or computing. For young women, the number is one out of 17. The rate of retention for both men and women from entry into a major to professional employment is about 60% in engineering and 40% in computing. Women start out so far behind in the number with appropriate majors that they can’t regain the ground.

2. The computing field has actually become unfriendly to women.

Before 1970, women took between 10% and 15% of computer science bachelor’s degrees. By the early 1980s, the number rose to 37%. However, the trend began to reverse in 1985. In 2013, 18% of bachelor’s degrees in computing were earned by women. Part of this decline is due to the fact that computer science became a male-identified field. But during the 1990s, hiring practices also began to favor men, according to the AAUW study, and “the creation of professional organizations, networks, and hierarchies” that supported the entry of men into the field helped pushed women out. In fact, as the study notes, once employed in the field, women are more likely to leave than men. They tend to suffer from isolation.

3. The atmosphere in the workplace matters a lot.

The dominating factor in women continuing in engineering is the work environment, the report found. “The women who stayed and who left were the same by many measures,” Corbett said. They had similar marital status, numbers of children, interest in engineering, and competence. “The women who stayed in engineering and were most satisfied had opportunities for training and development, clear paths of advancement, and were in environments with less uncivil [undermining] behavior on the part of colleagues and supervisors.”

Looking for solutions

Some schools have proven it is possible to attract women to engineering. For example, Harvey Mudd College was able to grow the computer science graduating class from 12% women in 2010 to 40% now. Such strategic changes as modifying the introductory course so that it emphasized the broad application of computing, developing a dual track for those with more and less exposure to computers at the start, providing opportunities for undergraduates to participate in research, and introducing female students to role models and examples of women in the profession have made the difference, according to the report.

Progress is far slower in the workplace. “We weren’t able to identify individual companies that were doing a great job [in technical staff diversity],” Corbett said.

However, some companies are pushing to improve the numbers. Google and BAE Systems are among those trying to address unconscious bias through a number of techniques, including training managers, broadening recruitment efforts, strengthening maternity and paternity benefits, and stripping resumes of information that could identify gender. E-commerce hosting company Etsy has taken various steps to increase the number of women on the technical staff. Broadening search methods and changing interview formats to make introverted candidates feel more relaxed helped increase the percentage of women programmers. Etsy has even funded immersive programming retreats, including needs-based living expense grants, to broaden the pool of applicants.