There are endless studies where girls are quoted directly complaining about how they are put off STEM because of their gender.
I'm not very familiar with sociology methodology, so excuse me if this is a stupid question - but is there any better evidence than self-reporting? People feeling a certain way may or may not equate to them being justified in feeling that way. For example, men are much more likely to suffer violent crime on the street than women are, but women self-report as being much more apprehensive of said crime. (http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0029265). I'm not claiming a feeling of apprehension has anything to do with self-selection for STEM subjects, just that I am wary of studies that rely on self-reporting to make objective claims about what is going on.