According to a 2011 study by the American Association of University Women (AAUW), about 40 percent of full-time faculty nationwide in 2005 were women, yet they made up only 22 percent of faculty in computer and information sciences, 19 percent of mathematics faculty and 12 percent of engineering faculty (the STEM fields). In 2005, the then President of Harvard University Lawrence Summers suggested that differences in math and science aptitude could explain part of this pattern. What reasons other than differences in aptitude could explain why women are under-represented in these fields?