[I]t is in large part because they are scientists that they do not want to believe these studies [on bias against women in STEM]. Scientists are supposed to be objective, able to evaluate data and results without being swayed by emotions or biases. This is a fundamental tenet of science. What this extensive literature shows is, in fact, scientists are people, subject to the same cultural norms and beliefs as the rest of society. The systemic sexism and racism on display every day in this country also exist within the confines of science. Scientists are not as objective as they think they are. It is an extremely destabilizing realization for someone whose entire career has been rooted in the belief in human objectivity.
Even more pernicious, however, is the understanding that results from reading these studies, the realization that those who have succeeded in science (and in many fields—the implications reach far beyond science) have not done so entirely due to their own innate brilliance. Statistically speaking, just being male will automatically give you a leg up. And no one wants to believe that they did not achieve their success, even in some small part, based on their gender or ethnicity.
Alison Coil on beliefs.