The lesson is modest. Build for the person who exists, not the one in the model. 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. We describe the departure from the model as an error. We might equally describe the model as a hopeful guess about us. It is tempting to call this irrational. It is more useful to ask what it is for.
What it gets right
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. The remedy is rarely exhortation. It is a change to the situation so that the easier choice is also the better one. The lesson is modest. Build for the person who exists, not the one in the model. Consider a good excuse. It is a small thing, and small things are where habits live.
