Tucker asked me if there was a name for the phenomenon where someone you don’t know asks you to watch over their possessions. I’m asked to do this a few times a day, for anything from books to laptops, at the ERC. (At the yuppie establishments that I also frequent, I’m not asked at all.)

If you’re worried that a total stranger might steal your belongings, why is it safe to ask a total stranger to guard them? There are at least two reasons.

The first reason is the asymmetry between your criteria, and those of a potential thief. Let’s assume some people are thieves, but not all. (If there aren’t any thieves present, it doesn’t hurt to recruit a guard, but you didn’t need to.) The chance of the person you chose being a thief is less than 1, but the chance of a thief choosing you (or your unattended belongings) is almost 1 — that’s our definition of “thief”: someone who is willing to steal from you. The chance of your picking a thief is therefore less than the chance of a thief picking you.

The second reason is social, not mathematical. Once you’ve made eye contact with someone and asked them for a favor — as long as you’re not obnoxious — then even if you did choose a potential thief, they’re less likely to steal from you. (This isn’t true for sociopaths, but even most thieves aren’t sociopaths.) Another way to put this is that interaction isn’t passive: asking someone for help changes the number of potential thieves in the room.