Many hypotheses have been posed about the reasons why there are fewer women than men identified as physicists, although most have not been rigorously tested (and some could not be in an ethical way.) Over time these hypotheses have shifted in nature. I will use the Halpern, Wai and Saw (2005) psycho-bio-social framework for analyzing gender differences to review recent findings that shed light on reasons why we might find fewer women in our physics classrooms and laboratories. I will present suggestions from this body of research about measures we might take to address the gender gap in physics, should we choose to. Finally, I will highlight resources available from AAPT, APS, and AIP- and hope to learn more about the efforts of the CAP.