Evolution of Cooperation - Prisoner's Dilemma

Prisoner's Dilemma

The prisoner's dilemma game (invented around 1950 by Merrill Flood and Melvin Dresher) takes its name from the following scenario: you and a criminal associate have been busted. Fortunately for you, most of the evidence was shredded, so you are facing only a year in prison. But the prosecutor wants to nail someone, so he offers you a deal: if you squeal on your associate – which will result in his getting a five year stretch – the prosecutor will see that six months is taken off of your sentence. Which sounds good, until you learn your associate is being offered the same deal – which would get you five years.

So what do you do? The best that you and your associate can do together is to not squeal: that is, to cooperate (with each other, not the prosecutor!) in a mutual bond of silence, and do your year. But wait: if your associate cooperates (that sucker!), can you do better by squealing ("defecting") to get that six month reduction? It's tempting, but then he's also tempted. And if you both squeal, oh, no, it's four and half years each. So perhaps you should cooperate – but wait, that's being a sucker yourself, as your associate will undoubtedly defect, and you won't even get the six months off. So what is the best strategy to minimize your incarceration (aside from going straight in the first place)?

To cooperate, or not cooperate? This simple question (and the implicit question of whether to trust, or not), expressed in an extremely simple game, is a crucial issue across a broad range of life. Why shouldn't a shark eat the little fish that has just cleaned it of parasites: in any given exchange who would know? Fig wasps collectively limit the eggs they lay in fig trees (otherwise, the trees would suffer). But why shouldn't any one fig wasp cheat and leave a few more eggs than her rivals? At the level of human society, why shouldn't each of the villagers that share a common but finite resource try to exploit it more than the others? At the core of these and myriad other examples is a conflict formally equivalent to the Prisoner's Dilemma. Yet sharks, fig wasps, and villagers all cooperate. It has been a vexatious problem in evolutionary studies to explain how such cooperation should evolve, let alone persist, in a world of self-maximizing egoists.