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.

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.

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.