There has been a lot of research on this in recent years, but I have also seen it personally. I am a female engineering student and the unconscious biases are very evident. Several girls I tutor have been told growing up that they do not need math, they are not good at math, the careers for girls don't involve math. Girls tend to then criticize their performance a lot more than the boys because they are looking for that reason why all the things they have been told are true. Most of the time in these situations, girls realize to get into the career they want they have to take at least one upper level math course and this scares them. They think they are incapable of completing the problem. If you don't think you can you never will. These girls have been prepared just as much as their counterparts, yet with their lack of confidence they perform lower in upper level courses. It has been a struggle for me as a tutor because I can teach material, I can explain material, but it is a lot harder to change a students perspective on the subject or on their own abilities.