The fundamental differentiator between early and late stage in those graphs is that there are far fewer positions as time moves on, in the 'higher' cohorts. It

The fundamental differentiator between early and late stage in those graphs is that there are far fewer positions as time moves on, in the 'higher' cohorts. It's not people just graduate along the path from A to Z - it's viciously competitive. Issues of maternity et. al. I think are just one important artefact of the nature of competition which for whatever reason I think is still going to favour men. Risk taking, hubris, aggression, physical endurance, combativeness, a certain kind if confidence, social expectations, possibly even the acceptance of those things by others - these things mean a lot at those later stages.