Every one of us runs the same shortcut, and every one of us believes we are the exception. Institutions know all this, of course, which is why the borrowed book is so rarely left to chance. 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 borrowed book is not that it exists but that it works, most of the time, for most people.
The ordinary case
The honest conclusion is uncomfortable for reformers: people are not waiting to be corrected. The mind that evolved for small groups and immediate dangers is now asked to manage a pension. It does its best. Institutions know all this, of course, which is why the borrowed book is so rarely left to chance. Nobody sets out to be governed by the borrowed book, yet it shapes an astonishing share of the ordinary day.
