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Cooperation evolves via reward strategy.

Birds do it, bees do it, even you and I do it: Instead of selfishly helping only ourselves or our close relatives, we sometimes choose to cooperate with genetic strangers. In the past decade, scientists have suggested that a strategy of "tit-fortat" guided the evolution of cooperative behavior in the animal world. A creature employing this scheme cooperates with a compatriot on a first encounter, then responds in kind to that animal's subsequent actions, whether cooperative or selfish.

But cooperation may have evolved largely in response to another rule of thumb, according to a report in the July 1 NATURE. A cooperative act that reaps rewards gets retained, in this theory; once it produces a loss, cooperation is abandoned for selfish behavior. The same pattern of "win-stay, lose-shift" governs selfish acts.

Computer simulations find that this strategy eventually outperforms tactics emphasizing tit-for-tat, selfishness, or cooperation alone, yet it facilitates a great deal of cooperation in its own right, assert zoologist Martin Nowak of the University of Oxford in England and mathematician Karl Sigmund of the University of Wien in Austria.

The late B.E Skinner and other "behaviorists" long argued that individual behavior is shaped by its consequences, particularly by rewards. Although Nowak and Sigmund take an analogous approach to the evolution of cooperation, they call their successful strategy "Pavlov" because it responds reflexively to rewards.

Nowak and Sigmund applied Pavlov and other tactics to the "prisoner's dilemma;' in which two players can either cooperate or act selfishly A player receives the most points for acting selfishly when the other cooperates, slightly fewer points if both cooperate, fewer still in cases of dual selfishness, and no points for cooperating when the other acts selfishly

In this game, a Pavlov player cooperates only if both participants chose the same alternative - either cooperating or not - in their prior encounter.

The researchers had previously found that a tit-for-tat strategy which forgives a few selfish acts by others helps boost cooperation (SN: 1/18/92, p. 39).

Pavlov, however, outperformed all other tactics in a series of computerized encounters intended to simulate biological evolution, Nowak and Sigmund contend. Strategies that yielded higher payoffs gained more influence in the overall simulation, whereas less successful tactics either lost favor or were eliminated.

Pavlov wielded little influence in the early stages of the trials but attained dominance by the end of the simulations, the investigators note. Pavlov's success stems from its tolerance of occasional random deceptions in the trials and its willingness to exploit consistently cooperative programs, they say

Strategies rooted in the Pavlov approach may indeed evolve naturally, writes zoologist Manfred Milinski of the University of Bern in Switzerland in an accompanying comment. Millnski and his co-workers find that stickleback fish that cooperatively check for predators in pairs and then encounter partners who flee during several consecutive predator checks react in Pavlovian fashion -- they alternately cooperate and flee on successive predator searches with defectors.
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Title Annotation:cooperative behavior among animals and humans
Author:Bower, Bruce
Publication:Science News
Date:Jul 3, 1993
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