ABSTRACT We use longitudinal data for the entire population of Norwegian PhD recipients over five decades to examine why only one-third 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 proportions 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 remaining imbalance is not caused by women dropping out, but rather by 14–18% slower average promotion rates calculated using hazard analysis. However, on average, women eventually catch up with men after about 20 years, although this differs by field. A similar hazard analysis for the US shows 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 US gender differences in advancement to full professorships and no eventual convergence. We suggest possible reasons for the differences between Norway and the US.