Two's In, to add from a Nav perspective, one CNI was castigated for bending the rules and allowing more extra hours per marginal stude than was budgeted for. The problem was that lower calibre students were being passed up the training system, where costs increased, before being chopped.
For the 'granny' stude the protracted training slope could extend in to first combat and risk aircraft and lives should the student not be up to speed.
As an examiner I was once presented with an operational crew where the navigator was below average and the squadron hoped that we would have wielded the axe. In the event my boss refused to be the hatchet unit and left that to the squadron. I have no doubt that that crew would have been combat ineffective and blame the CO for not facing up to his responsibilities.
Back to nature-nurture, I believe looking at different branches is also revealing. Aircrew, I believe, appear as a body to be less risk averse than engineers. The difference is that aircrew control their own risk whereas engineers control risk of others. Does this risk appetite vary as each becomes more indoctrinated in their roles?