Generous players: game theory explores the Golden Rule's place in biology

Science News,  July 24, 2004  by Erica Klarreich

• E-mail
• Print
• Link

In the 1970s, the father of evolutionary game theory, the late John Maynard Smith, studied the snowdrift game under a different guise, the hawk-dove game, that used a different story line but modeled the same system of rewards and punishments. Maynard Smith and the late George Price showed that cooperators and defectors coexist stably in a mix whose proportions are determined by the payoffs of the particular game. This contrasts with the prisoner's dilemma, in which cooperators die out except in special circumstances.

Curiously, the phi6 RNA viruses, which play the prisoner's dilemma in certain conditions, play the snowdrift game in others. In 2001, Turner and Chao found that if the cooperative viruses evolve in isolation before being mixed with defectors, they become so good at making the molecules needed for reproduction that they thrive even when the defectors steal molecules. Then, as in a snowdrift game, even if the cooperators do all the hard work, they're not necessarily cut down by the exploitative defectors.

NEIGHBORHOOD CONCERNS Plants and many microbial colonies and animal populations interact only with nearby organisms. In their recent Nature paper, Hauert and Doebeli used computer modeling to analyze what happens in the snowdrift game when players are limited to their immediate neighbors. The researchers found to their surprise that neighbor-only interactions are detrimental to cooperation in the snowdrift game.

In the prisoner's dilemma, by contrast, cooperation gets a big boost when players interact only with their neighbors. In 1992, Nowak and Robert May of Oxford University in England showed that for certain payoffs of the game, spatial structure permits cooperators to survive in protective clumps instead of dying off'. By clustering, they insulate themselves from the many defectors in the population at large.

The difference between the games' outcomes arises because in the snowdrift game, but not in the prisoner's dilemma, it's in a player's best interest to be the opposite of his neighbors. That distinction changes the patterns of cooperators and defectors that arise. In Hauert and Doebeli's computer simulations of the snowdrift game, cooperators tend to form long tendrils, instead of the clumps observed in the prisoner's dilemma. The upshot is that the cooperators are exposed to exploiters even more than if the population mixed freely.

"There's a common belief that any form of spatial structure will promote cooperation," Hauert says. "We're challenging that."

Nowak says that Hauert and Doebeli are right to encourage game theorists to examine the snowdrift game more closely. "I've always had the perspective that the prisoner's dilemma is the only game where you could possibly talk about cooperation," he says. "Their approach has made me revise that perspective."

Ultimately, a better understanding of the interplay between cooperation and exploitation could help explain the emergence not just of cooperation but also of life itself. After all, life owes its origins to primeval acts of inanimate cooperation, in which RNA, proteins, and other molecules banded together to form cells.