The progress made by American women in higher education has been impressive. In 1970 a higher fraction of men than women completed a college education. During the following four decades, the gender gap in education not only decreased but became negative. However, this was not always so. The pre-1970 period (1950-1970) was characterized by an opposite trend: the gender gap strongly increased. This paper develops a model to quantify, within a unified framework, the relative importance of supply and demand forces on the rise and fall of the U.S. gender gap in education. Specifically, I build and calibrate an assignment model for the U.S. economy with endogenous human capital accumulation of women and men, where three different sources of education gains exist: (1) supply shifts, (2) a within-sector skill-biased technical change (SBTC), and (3) the creation of new high-skill services/sectors. I find that asymmetrical supply shifts by gender were the major force behind, first, the increase in the gender gap during the pre-1970 period and, second, the decrease in the gap during the post-1970 period. The empirical results show positive supply shifts for men but not for women during the pre-1970 period, and negative supply shifts for men together with no changes in supply for women during the post-1970 period. I discuss some explanations for the dissimilar behavior of women's and men's supply during both periods.