Have you ever been to a retail store and seen this sign on the register: “Your purchase free if you don’t get a receipt”? You almost certainly didn’t see it in an expensive or high-end store. You saw it in a convenience store, or a fast-food restaurant. Or maybe a liquor store. That sign is a security device, and a clever one at that. And it illustrates a very important rule about security: it works best when you align interests with capability.

If you’re a store owner, one of your security worries is employee theft. Your employees handle cash all day, and dishonest ones will pocket some of it for themselves. The history of the cash register is mostly a history of preventing this kind of theft. Early cash registers were just boxes with a bell attached. The bell rang when an employee opened the box, alerting the store owner—who was presumably elsewhere in the store—that an employee was handling money…