I lock my bike, but often I don’t bother to lock my helmet. Instead I buckle it around the lock so that the casual onlooker would guess that it too is secure.

A thief, however, is no casual onlooker. If thieves were out to steal bike helmets, they’d have stolen mine long ago. This gets me thinking about what makes it rational to be so insouciant about my helmet.

Let’s say the bike costs 300 pounds and the helmet costs 30. Then loss of the helmet means just 1/10th the pain of loss of the bike.

But because it’s worth less, the helmet is also less likely to be stolen! Let’s say, 1/10th as likely. So we have a quadratic effect in play: the expected loss from leaving my helmet unlocked is 1/100th that of leaving my bike unlocked.

[19 December 2016]