In the studies we have, the effect is modest, reliable and almost entirely ignored by the people it describes. Institutions know all this, of course, which is why the last biscuit is so rarely left to chance. The mind that evolved for small groups and immediate dangers is now asked to manage a pension. It does its best. 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 ordinary case
The remedy is rarely exhortation. It is a change to the situation so that the easier choice is also the better one. Notice how rarely anyone is argued out of the last biscuit, and how often they are simply moved past it by a change in circumstance. The honest conclusion is uncomfortable for reformers: people are not waiting to be corrected. The textbook treats kindness as a preference to be revealed. In practice it is a story we tell ourselves after the fact.
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 lesson is modest. Build for the person who exists, not the one in the model. The strange thing about the last biscuit is not that it exists but that it works, most of the time, for most people. The evidence here is thinner than the confident commentary suggests, and the honest position is a shrug with a direction of travel.
