Institutions know all this, of course, which is why the round number is so rarely left to chance. The lesson is modest. Build for the person who exists, not the one in the model. The evidence here is thinner than the confident commentary suggests, and the honest position is a shrug with a direction of travel. The mind that evolved for small groups and immediate dangers is now asked to manage a pension. It does its best.
A better question
We describe the departure from the model as an error. We might equally describe the model as a hopeful guess about us. Notice how rarely anyone is argued out of the round number, and how often they are simply moved past it by a change in circumstance. The usual story is that we are careless calculators. The better story is that we are careful about the wrong things. The strange thing about the round number is not that it exists but that it works, most of the time, for most people.
