When the cashier hands you an extra twenty, you find out your price. Make sure it’s higher than twenty.

Honesty gets tested by accidents rather than by grand dilemmas. An extra note in your change, a self-checkout that missed an item, a wallet left on a seat: nobody is watching, the amount is small, and what you do next reports your actual price rather than your stated one. The number tends to be lower than people expect until they have been tested a few times.

Decide in advance, while nothing is at stake, so there is no debate in the moment. Hand it back the first time, and notice how little it cost.