Factors affecting gender disparity in introductory physics

In America, the disparity between the performance of women and men in physics is a matter of common concern; however, it is poorly understood. Is this disparity observed among physics majors at elite institutions as well as non-majors in one-year introductory courses? Does this disparity depend on the pedagogical approach used to teach the class? The high school physics or mathematics background of the students? The gender of the instructor?