The lesson is modest. Build for the person who exists, not the one in the model. The point is not that people are foolish. The point is that habit is expensive, and we economise on it like everything else. 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. The textbook treats habit as a preference to be revealed. In practice it is a story we tell ourselves after the fact.
Why it persists
What looks like habit from the outside is usually something simpler from the inside: tiredness, a deadline, a wish not to look foolish. Nobody sets out to be governed by the weather forecast, yet it shapes an astonishing share of the ordinary day. It is tempting to call this irrational. It is more useful to ask what it is for. There is a version of this essay that ends with a policy prescription. This is not that version; the mechanism is interesting enough on its own.
