HAVE you have ever seen a sticker on the back of a tradesman's van saying “No tools or valuables stored in this van over night”? I've seen them in London for a while and started wondering whether they actually deter thieves or simply draw attention to the fact that the driver is a nervous tool owner.

My reasoning goes as following. First think of the van owner: there is a small cost to obtaining a sticker but this is basically inconsequential. There is a large cost to the owner of removing his heavy tools every night and taking them to a safe place. Even if he intends to, he may forget or chose not to on any given night. Now consider the thief: there is a cost every time he breaks into a van—the police might come and catch him plus there is the effort of the break in itself—therefore he chooses his target vans carefully.

There are two types of van owner: those with tools and those without. Both might conceivably put a sticker up. Those with tools fear for their safety (and worry that they may forget to remove them). Those without might worry their van might be broken into by those seeking tools. A break-in is costly to both types.

Consider the thief's reaction to this. Most if not all sticker owners are tool owners, and nervous ones (which suggests their tools may be valuable) at that. The sign may have the added effect of lulling those displaying it into a false sense of security. On any one night a decent percentage of them may have forgotten to remove their tools or chosen not to out of laziness. A good strategy might therefore be to target only vans with deterrent stickers as the success rate will be higher amongst this group.

I ran my musings past Jeff Ely, a game theorist at Northwestern University. He added that the one effect you can count on is that the sticker brings to mind "tools" and "removing tools". There are different types of thief as well. Would-be thieves who might never have thought that there might be tools in the van are now cued to the opportunity. But equally, those who might have never considered that tools could be removed must now consider the possibility. Mr Ely says the sticker is definitely a negative because the first category of thief is now more likely to break in while the second category will pause for a moment, realise that even though it was possible to remove the tools, it is probably not something the van owner would actually do and will go ahead and break in.