As the queues formed down the street outside branches of Northern Rock this week, it was obvious enough to a game theorist what was going on: people had decided to hunt rabbits. Bear with me – this will make sense in a moment.

Game theory is the study (by economists, mathematicians, biologists and others) of situations where what you do may affect what I choose to do, and what I do may affect what you choose to do. The theory is big on catchy stories with memorable names, but ultimately it is all about mathematical representations of interactive decisions, called “games”. The most famous game of all is the “prisoners’ dilemma”, in which two prisoners must each decide whether to plea-bargain by giving evidence against the other.

But another game, the “stag hunt”, languishes in relative and undeserved obscurity. In the stag hunt, two hunters must each decide whether to hunt the stag together or hunt rabbits alone. Half a stag is better than a brace of rabbits, but the stag will only be brought down with a combined effort. Rabbits, on the other hand, can be hunted by an individual without any trouble.

There are two rational outcomes to the stag hunt: either both hunters hunt the stag as a team, or each hunts rabbits by himself. Each would prefer to co-operate in hunting the stag, but if the other player’s motives or actions are uncertain, the rabbit hunt is a risk-free alternative.

Northern Rock’s customers have decided to hunt rabbits. If they had confidence that other depositors would stay with the bank, there’s not much doubt that their money would be safe. When they lost that confidence, the stag hunt was abandoned. And despite all the talk about panic, it was abandoned for perfectly sensible reasons.

Continued at ft.com, subscription free.