Put a number on expertise and it becomes a target; put a target on it and the number stops meaning what it meant. The measure that matters is not what people say about expertise but what they do when nobody is scoring. A better account of expertise would begin with the ordinary case rather than the laboratory one. Economists have spent decades modelling expertise as a quantity. It behaves more like a weather system.
Where the model breaks
Put a number on expertise and it becomes a target; put a target on it and the number stops meaning what it meant. Understanding people turns out to be a more urgent task than changing them, and a great deal harder. 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. If you want to see expertise at work, watch a queue form where there is no reason for one.
