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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 -----BEGIN PGP SIGNATURE----- Version: 2.6 iQCVAgUBLmJSsEGHwsdH+oN9AQGIAAQAkT6GC1xOdmCh5Zp7LU17oKRH7WAqeYoK 6FypHPqfUK688uFUAUz61MhGaMkr9ZoCcnRdsmejOGq9zQ9sW6D3SnGvTtkgGyGD zNjle57RVxG8sqkaei8kKszCyVIxZfms2RkdrmQyC/GHwAo9i/5yOszdqFotWfVJ HRe05Pfrano= =zsiI -----END PGP SIGNATURE-----