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

Science News, July 24, 2004 by Erica Klarreich

Charles Darwin's theory of natural selection seems to describe a brutal world in which creatures compete ruthlessly to promote their own survival. Yet biologists observe that animals and even lower organisms often behave altruistically. A vervet monkey who spots a leopard, for instance, warns his fellow monkeys, even though the call may attract the leopard's attention to the individual. A vampire bat that has hunted successfully shares nourishing blood with a fellow bat that failed to find prey.

Such behavior is obviously beneficial for the species as a whole. However, natural selection postulates that successful organisms act to propagate their own genes. If selfish animals can take advantage of more-generous peers, how has any generous behavior survived the mill of natural selection? Darwin himself pondered this puzzle. Focusing on human evolution, he wrote in 1871 that "he who was ready to sacrifice his life, ... rather than betray his comrades, would often leave no offspring to inherit his noble nature."

Somehow, the altruistic behaviors observed in the wild must benefit the giver as well as the receiver. However, pinpointing how this works in animal populations is a huge challenge. In most cases, it's impossible to measure precisely how an animal's cooperative behavior affects its chances for survival and reproduction.

Now, theoretical research is starting to fill in the picture of how cooperation may survive natural selection. Some of the most illuminating ideas are coming from game theory, the field of mathematics that studies strategic behavior in competitive situations.

For decades, game theorists' basic paradigm for the puzzle of cooperation has been the scenario called the prisoner's dilemma, in which each player has a powerful incentive to exploit the other. The game is set up so that cooperation is best for the group, but each player individually does better by taking advantage of the other. A growing body of mathematical analysis and computer modeling now suggests that in many circumstances, cooperators can survive in the prisoner's dilemma. In the April 8 Nature, researchers argue that, under certain conditions, a cooperator can infiltrate, and eventually take over, a population of cheaters.

Meanwhile, other game theorists are arguing that the prisoner's dilemma isn't the be-all and end-all of cooperation test beds. In the same issue of Nature, researchers highlight another set of interactions, called the snowdrift game, in which players have incentives both to cooperate and to exploit each other. The new analysis of the snowdrift game challenges some accepted wisdom about which environmental factors encourage cooperative or exploitative behavior.

SELFISH STRATEGIES In the prisoner's dilemma, the police are separately interrogating two accomplices. Each criminal has two options: to cooperate with the other by keeping quiet or to defect by squealing on the other. If both cooperate, they'll each receive a 1-year sentence. If each incriminates the other, they'll both get 5 years. But if one cooperates and the other squeals, the cooperator will land a 10-year sentence, while the squealer will get off with only 6 months in jail.

Chase through the options, and you'll find that no matter what course one prisoner chooses, the other will do better by defecting. So, if the two players are perfectly rational, both will inevitably squeal.

The game neatly encapsulates the cooperation paradox: Even though cooperation is the best plan, it fails to be adopted, since cheating benefits the individual.

Although the prisoner's dilemma scenario may seem artificial, many interactions of animals and other organisms may have a similar structure of rewards and penalties, some biologists and game theorists argue. In biological versions of the prisoner's dilemma, organisms are competing not for shorter prison sentences but for increased fitness and reproductive success.

It's not that a paramecium, say, mulls over possible strategies. "You don't need to assume that the players of a game are rational and are bent on out-thinking each other," says Karl Sigmund, a game theorist at the University of Vienna in Austria. "They just have to follow their inbuilt programs."

What's needed is for strategies to be predetermined by an organism's genes and inherited from one generation to the next. Then, if one strategy outperforms the others, the individuals using that strategy will tend to have more offspring, who will also follow the superior strategy. After many generations, the weaker strategies will have been weeded out, and the players will be using the strategies that rational thinkers would have come up with.

In fact, a variation on the prisoner's dilemma can take place with participants--RNA viruses--about as far removed from high-level reasoning as imaginable. In 1999, Paul Turner of Yale University and Lin Chao of the University of California, San Diego mixed two different strains of a virus called phi6. They had chosen one strain that abundantly manufactures the molecules the virus needs for reproduction, and another that puts fewer resources into making these molecules and instead grabs the molecules that the other viruses make. "They are cheaters, who exploit the viruses doing the heavy lifting," Turner says.