Put a number on certainty and it becomes a target; put a target on it and the number stops meaning what it meant. The mind that evolved for small groups and immediate dangers is now asked to manage a pension. It does its best. If you want to see certainty at work, watch a queue form where there is no reason for one. The strange thing about the to-do list is not that it exists but that it works, most of the time, for most people.
Three kinds of mistake
A better account of certainty would begin with the ordinary case rather than the laboratory one. The measure that matters is not what people say about certainty but what they do when nobody is scoring. Ask most people why they did what they did and they will give you a reason; ask them again a week later and the reason will have quietly changed. Put a number on certainty and it becomes a target; put a target on it and the number stops meaning what it meant.
The honest conclusion is uncomfortable for reformers: people are not waiting to be corrected. The usual story is that we are careless calculators. The better story is that we are careful about the wrong things. A pattern this consistent is not a character flaw. It is a design feature, and design features can be worked with. The point is not that people are foolish. The point is that certainty is expensive, and we economise on it like everything else.
