What looks like expertise from the outside is usually something simpler from the inside: tiredness, a deadline, a wish not to look foolish. It is tempting to call this irrational. It is more useful to ask what it is for. Put a number on expertise and it becomes a target; put a target on it and the number stops meaning what it meant. Every one of us runs the same shortcut, and every one of us believes we are the exception.
Three kinds of mistake
Consider the borrowed book. It is a small thing, and small things are where habits live. A pattern this consistent is not a character flaw. It is a design feature, and design features can be worked with. A better account of expertise would begin with the ordinary case rather than the laboratory one. It is tempting to call this irrational. It is more useful to ask what it is for.
