Believing that you can make it is an important ingredient in success. For women, that confidence is harder to come by when they are pursuing science, technology, engineering or math. A study by the Mathematical Association of America found that of those in Calculus I who initially intended to go on in science, technology, engineering or math, 

... if we restrict our analysis to just those who are earning an A or B in the course ...  18 percent of the women, but only 4 percent of the men, believed they did not understand calculus well enough to continue.

I’ll bet the phenomenon of women who are objectively doing equally well as men having much less confidence than men extends to many other moments in education in technical subjects--including training in economics. Professors and other instructors can do a lot of good by bolstering the confidence of female students who are in fact doing well in a class or in independent research. 

In my own experience, I have sometimes been quite surprised at how much my expressions of confidence in a female student in economics have meant to that student--as if that student were in a parched desert for such expressions of confidence, despite what seemed to me excellent skills. 

In addition to discouragement, low confidence in oneself also causes other people to underestimate one’s skills. It is quite difficulty to know how skilled someone is, but typically quite easy to tell how skilled they think themselves to be. So people use a job candidate’s opinion of herself or himself as a shortcut for judging her or his skills. 

For those who need to come across as more confident I highly recommend the weekend personal growth workshops conducted by Landmark Education Corporation, beginning with the Landmark Forum. In my view, almost everyone entering the dissertation writing and then job-hunting phases of getting a PhD in economics should do the Landmark Forum because of how much it will help the psychology of being able to focus on dissertation research and then the psychology of self-presentation for getting a job. I am sure that the same advice would apply to students in many other fields, at many stages of education.