In 1960, there were 1.55 males for every female undergrad, but by 2003 there were 1.3 females for every male undergrad. What happened?
In The Homecoming of American College Women: The Reversal of the College Gender Gap (NBER Working Paper No. 12139), authors Claudia Goldin, Lawrence Katz, and Ilyana Kuziemko offer some explanations for the change.
Summary here.