From: John Douceur <johndo@microsoft.com>
Date: Mon, 29 Aug 94 14:38:29 PDT
To: cypherpunks@toad.com
Subject: iterated prisoner's dilemma
Message-ID: <9408292139.AA06676@netmail2.microsoft.com>
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>From: Hal  <hfinney@shell.portal.com>
>Date: Monday, August 29, 1994 12:03PM

>Another argument sometimes advanced in favor of trustworthy escrow
>agents is the "iterated prisoner's dilemma".  This refers to Axelrod's
>simulations of computer program agents which repeatedly interacted in
>a simple "prisoner's dilemma" game which captures much of the essence
>of the trust relationship (see his book "The Evolution of Cooperation").

>His results generally have consistently shown that agents which are
>never the first to "cheat" in a relationship do better than those
>which try to take advantage of their counterparts.
 . . .
>Axelrod's tournaments were predicated on the implicit
>assumption of an indefinite number of interactions.  (This is my
>recollection; I'd be interested in whether experiments have been tried
>with a known fixed number of interactions, and the agents knowing how
>many more there were.)  It had long been recognized (pre-Axelrod) that
>the prisoner's dilemma might reach a stable cooperative solution with
>multiple interactions, but that this becomes unstable if the parties
>know that they are reaching the end of their interaction period.

Axelrod's second tournament had a variable number of interactions,
precisely to defeat penultimate-interaction attacks.  He added this
specifically because his first tournament had a fixed and known number
of interactions, and several programs took advantage of it.  However,
even in the first tournament, the "nice" programs did better than the
"mean" programs, and Tit-for-Tat was the winner.

I suppose this doesn't prove much, insofar as a Tit-for-Tat-but-
Screw-Em-on-the-Last-Round program would probably have come in first
had it been entered.  Even so, I expect that the marginal increase in
score over Tit-for-Tat would have been vanishingly small for a large
number of interactions.

JD


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