For the sake of argument let’s take on the plain utilitarian case for waterboarding: in return for the suffering inflicted upon a single terror suspect we may get information that can save many more people from far greater suffering. At first glance, authorizing waterboarding simply scales up the terms of that tradeoff. The suspect suffers more and therefore he will be inclined to give more information and sooner.

But these higher stakes are not appropriate for every suspect. After all, the utilitarian cost of torture comes in large part from the possibility that this suspect may in fact have no useful information to give, he may even be innocent. When presented with a suspect whose value as an informant is uncertain, these costs are too high to use the waterboard. Something milder is preferred instead like sleep deprivation.

So the utilitarian case for authorizing waterboarding rests on the presumption that it will be held in reserve for those high-value suspects where the trade-off is favorable.

But if we look a little closer we see it’s not that simple. Torture relies on promises and not just threats. A suspect is willing to give information only if he believes that it will end or at least limit the suffering. When we authorize waterboarding, we undermine that promise because our sleep-deprived terror suspect knows that as soon as he confesses, thereby proving that he is in fact an informed terrorist, he changes the utilitarian tradeoff. Now he is exactly the kind of suspect that waterboarding is intendend for. He’s not going to confess because he knows that would make his suffering increase, not decrease.

This is an instance of what is known in the theory of dynamic mechanism design as the ratchet effect.

Taken to its logical conclusion this strategic twist means that the waterboard, once authorized, can’t ever just sit on the shelf waiting to be used on the big fish. It has to be used on every suspect. Because the only way to convince a suspect that resisting will lead to more suffering than the waterboarding he is sure to get once he concedes is to waterboard him from the very beginning.

The formal analysis is in Sandeep’s and my paper, here.