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Selfishness in P2P Networks

U C B E R K E L E Y C O M P U T E R S C I E N C E

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Introduction to Game Theory

Prisoner DilemmaTwo members A, B of a criminal gang are arrested and put in separate solitary confinement. The prosecutors lack sufficient evidence and hope to convict them of at least to a lesser charge to get both sentenced to one year in prison. But they offer each prisoner a bargain. Each prisoner is given the opportunity either to: betray the other, or confess the crime by remaining silent. The offer is:

If A and B each betray the other, each serves 2 years in prisonIf A betrays but B confesses, then A will be free but B will get 5 years (and vice versa)If A and B both confess, then both of them will only serve 1 year in prison

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Prisoner’s dilemma

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What will A and B do?What is the lesson?

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Prisoner’s dilemma

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Because betraying the fellow criminal offers a greater reward than cooperating (i.e. confessing), all purely rational self-interested prisoners will betray the other, and thus bothWill get 5 years instead of 1 year.

Selfishness is often considered rational, but there is a price.