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Lock Your Doors

J. Puma asks why people lock their doors:

Okay, so, when you're locking your doors, you are acting under the assumption that *at least* 51% of the individuals who might pass your house will attempt to enter. You're locking the doors because you're pretty sure that more people would try to break in and steal stuff or harm you than not. You're assuming bad intentions in at least 51% of the population of your neighborhood, town, city.

... Wanna know what the actual rate is? According to Bureau of Justice statistics, the actual chance (as of 2002) that someone would break into your house and steal your stuff is . . . 2.77%.

Where is he getting this 51% figure? To do risk calculations, you multiply the chance of each outcome by its magnitude. So let's say I would experience 10,000 units of harm if someone did come into my house and steal my stuff. At about a 3% chance of that happening, that's an expected value of -300 units if I leave my door unlocked. The other option is locking the door. Let's say I experience 10 units of harm from locking my door due to the hassle of having to get my keys out every time I come or go. Since I'll get that harm every time, the chance of it happening is 100%, making the expected value -10. So it's -300 versus -10 -- clearly locking the door is the smart choice, given even that small chance of robbery. Of course, the numbers I plugged in are ex recta, but they're in the right ballpark (being robbed is far worse than having to lock your door), and they illustrate the right way to do these calculations.

But really, the expected value of locking your door is -310, because the 3% statistic that Puma offers is the chance of being robbed in a society where door-locking is common. It doesn't tell us how much higher the rate would be if people routinely left their doors unlocked. It seems quite plausible that locked doors deter people from burglary by making it not worth their while to try to break in (there's at least one guy who's been seen walking around the Union Station parking lot testing car doors, looking for ones that are unlocked). They probably deter a good deal of crime just in the planning stages -- people don't seriously consider stealing because they figure the doors will be locked. So if you leave your own door unlocked and have no problems, you may just be free-riding on the door-locking of your neighbors, which creates the impression in potential burglars' minds that all doors are likely to be locked. Puma seems to suggest that there aren't many people in this middle group who would be deterred by a lock -- either you're a hardened criminal who will smash the window, or you're a law-abiding citizen who wouldn't take anything even if the door was wide open. I find it hard to believe that the increase in burglary due to widespread door-unlocking would be so small as to make the expected value of being robbed after not locking your door less than the expected value of locking your door plus the expected value of being robbed anyway. The BJS statistics seem to back me up on this, as rates of theft (taking stuff when you have a legal right to be in the house) are four times higher than those of burglary (when you have to enter illegally). I find it hard to believe that moral qualms about trespassing, rather than the physical barriers to entry presented by things like locks, explain all of that difference.

Puma also suggests that the expected outcome of door-locking is really much higher than the small allowance I made for the hassle of getting your keys out. He argues that it cultivates a culture of distrust that is extremely damaging to ourselves and others. There is something to that. But in my mind there's not as much there as Puma thinks. Part of the reason is the habitual nature of door-locking. If anywhere, then here in Main South Worcester, the pragmatic calculation of door-locking weighs in favor of it. So I always lock my house and car. But the other weekend I was visiting a friend in rural Vermont, and I still locked my car. Was I terrified that her family might take my car, or that there was some burglar prowling the woods? Of course not. Reaching for the lock was automatic, a habit divorced from all feelings of distrust. Even here in Worcester, locking is largely habitual. I never see a neighbor or passerby (an adult one, at any rate) and wonder whether they might want to make off with my car or computer.

It's not merely the increased distrust (however justified it may be) of modern society that's the issue here. It's also the increased importance of private property. On the one hand, the negative utility of losing our stuff is higher than it would have been for people in the past, because we're more attached to it. On the other hand, we draw a strong line between the private and public spheres. Locking the door is not just a pragmatic precaution. It's also a ritual of sorts, signalling and reinforcing the demarcation between the home and the outside world.


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