We use longitudinal data on the entire population of Norwegian PhD recipients over five decades to examine the reason only 1/3 of full professors in Norway are women, despite gender balance among current PhDs. We find that 90% of the lower female representation is due to lower female shares in earlier PhD cohorts, increasing sizes of cohorts, and decreasing overall rates of promotion, which together we call “compositional/historical factors.” We find that the small remaining imbalance is not caused by women dropping out but rather by 15% slower average promotion rates calculated in hazard analysis. However, women eventually catch up with men after about 20 years. We conduct a similar hazard analysis for the US and find that women doctorates are less likely than men to enter tenure-track academia although more likely to enter non-tenure-track academia. This leads to larger gender differences in advancement to full professorships and no eventual convergence. We suggest possible reasons for the differences between Norway and the US.