A grade earned in an introductory college course is a pretty good predictor as to whether someone is going to major in that subject. HOWEVER, men and women respond very differently to that grade. Nice graphic with an example here.

This graphic comes from a study by Harvard professor Claudia Goldin (click for an article by her on this topic).

It shows, by grade attained in an introductory economics class, which gender is more likely to continue on and major in Economics.

When BOTH men (blue bar) and women (red bar) receive an "A" in the class, women are slightly more likely to go on and major in the subject.  However, as the grade earned gets lower women flee the major at a much higher rate than men do.

In other words, men are undeterred by a lower grade in deciding to major in economics and women quickly have second thoughts and move on to something else.

Economics suffers from a deficit of females in the major similar to the STEM majors.

This excellent analysis HERE at the Washington Post goes deeper into the potential reasons.  I highly recommend it if you are interested in the issue.

Source: Washington Post