• 57% of bachelor's degrees are earned by women, but only 18% of computer science degrees are earned by women
• of all physics professors in the USA, only 14% are women
• women hold less than 25% of STEM jobs in the USA
• 28% of the world's researchers are women
• STEM statistics for female visible minorities are even lower
• in 2012 Yale published a study that proved physicists, chemists, and biologists are likely to view a young male scientist more favourably than a woman with the same qualifications, and that there is a major gender pay gap for female scientists IF they somehow manage to get hired instead of their male co-applicants  

1. Social Anxiety or Herd Mentality. In the Yale study, many female university students cited high school stereotypes as a major reason for why they chose to enrol in liberal arts courses instead of STEM classes. The female students were anxious to apply to STEM university programs when they knew they could very well be one of the few women in the entire program, let alone in their courses. Let's look at a Canadian example: in Waterloo's Computer Science program, women made up only 12% of the program in 2011. That's a significant decrease in the female to male ratio when women made up over 18% of enrolment in 2001. And that's despite Waterloo's valiant efforts to support women in computer science. It is very hard to encourage more women to pursue STEM programs and break the trend when they must trust other women to also apply at the same time so that they won't feel isolated in a male-dominant space.
2. Lack of Support and Role Models. This is mostly self-explanatory, but if girls aren't encouraged to pursue their STEM dreams, they might not even stop to consider STEM as a viable opportunity for their future. Teachers, professors, institutions and society need to help open the STEM door for women, and then they can make the decision whether or not they want to walk through. Confidence is another big part of the equation. For whatever reason, girls are less confident in math and science than boys. Studies have shown that when told that men score better in math tests than women, women tend to score worse; but when told that isn't true, both genders score equally well.
3. Bias. Think about it from the point of view of a female physics PhD candidate: when you know for a fact that the tech industry is biased towards your male counterparts, that you'll face a greater degree of sexism in the workforce, that you'll probably be the only female scientist in your entire department, that you won't get pay equity... why would you try to pursue a job in STEM?

The irony is that up until university, high school girls in STEM classes perform exceedingly well: in most studies, it is revealed that they attain much higher grades than their male peers. Yet, researchers are witnessing a huge drop off with hardly any of these high-performing high school girls applying to STEM undergraduate programs.