The lesson is modest. Build for the person who exists, not the one in the model. Understanding people turns out to be a more urgent task than changing them, and a great deal harder. The textbook treats regret as a preference to be revealed. In practice it is a story we tell ourselves after the fact. 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 cost of being right
It is tempting to call this irrational. It is more useful to ask what it is for. The point is not that people are foolish. The point is that regret is expensive, and we economise on it like everything else. None of this is new. Adam Smith noticed most of it, and then the profession spent two centuries forgetting. In the studies we have, the effect is modest, reliable and almost entirely ignored by the people it describes.
