Game theory has become a useful way to evaluate strategies for survival in evolution scenarios. In a new study, scientists set up a model where human players engage with each other and compete for resources, and can change their strategies for doing so in various ways. They found that as more rounds of the game were played, the human players developed a tendency to imitate the best player, causing the players as a group to tend to play the game the same way. This implies that in evolution, as one member of a species enjoys more and more success, its methods become hard to ignore for the others, which will eventually follow its lead.

Evolution is a popular application of game theory, in particular to see how trends and strategies for survival spread among a group. In this situation, survival isn't based on an absolute indicator of fitness, but instead on evaluating how others in the same community are faring based on their actions. Because of this, it is important to figure out how certain survival strategies come to be adopted. One popular method of representing this scenario is with a situation called "the prisoner's dilemma."

The prisoner's dilemma is often used in game theory when applied to evolution. It's usually described something like this: two arrested criminals who are in cahoots with each other are taken into separate rooms, each with a cop, and are offered a plea bargain. If the first informs on their confederate, the first will go free and the confederate will receive the maximum sentence, and vice versa. If they both rat each other out, both receive a large, but less than maximum, sentence, but if both remain silent, they each receive a small sentence. This situation can be altered to give money payouts instead of prison time, to make quantifying easier. It should be noted that the highest individual payoff goes to someone who gets away with informing on a trusting partner, and the highest total payoff goes to two partners who cooperate and don't inform on each other.

In the experiment, researchers set up a 4x4 lattice with players in the prisoner's dilemma at each lattice point. Each player played against only his four immediate neighbors in four one-on-one games. The players could then evaluate their performance by comparing themselves to their neighbors, seeing who among the group received the largest amount of money, and could evaluate the performance of their neighbors over multiple matches. Each player is also modeled as human, rather than theoretical, and so is capable of using mixed strategies or changing strategies over time and does not always make the best strategical choice.

The model's creators found that as more and more rounds of the game were played, players were increasingly likely to adopt the strategy of the best performing player. In early rounds most of the groups were heterogeneous—some players defected, and others cooperated with each other—and about 40 percent of the players used strategies not based on that of the best player. As time went on, the groups became increasingly homogeneous, with either all players defecting or cooperating with each other.

The odds of adopting a random (not-imitating-the-best) strategy reduced over 25 rounds to about 18 percent, with a significant increase in players imitating the best player in a homogeneous group. Researchers found that the odds of switching to following the best player went up as the payoff difference increased; eventually, the wealth of a player using a certain strategy became too hard to ignore.

This sounds like an obvious result, but it's possible to picture other strategies that are not so focused on one player prevailing—for example, surveying the whole group at once and using the strategy of the majority, or attempting to guess the next step of each player based on their play history and choosing the next move accordingly.

Overall, the persistence of random strategies, which were considered mutations in this model, were much higher in the beginning than in typical theoretical models. Even so, players were increasingly likely to abandon them and do what the most successful player did as she continued to be successful. In terms of evolution, this is the equivalent of selection, where organisms trying to survive stop what they're doing and start imitating the organism that's having the best luck in attracting the opposite sex.

There is still some contention over whether models should be so small as the one in this experiment. The study argues that our predecessors only concerned themselves with their own small social groups, and our behavior may be adapted to that situation. Its researchers note that a good next step would be to model systems on a more complex level that categorizes strategies in a more specific way than "imitation of an individual" and "everything else," and to study them one both heterogeneous and dynamic networks.

PNAS, 2010. DOI: 10.1073/pnas.0912515107

CC licensed photo courtesy of Thunderchild tm