Sunday, September 5, 2010

Evolving cooperative behavior with neural controllers

In a computer experiment, we have investigated the evolution of cooperative behavior in multi-player games. Players were randomly mixed into groups and had the chance to increase their investment by paying money into a pot where it was multiplied. However, the payout money was evenly distributed to all of the players regardless of their contribution. So a freerider could get money without paying into the pot as long as some others did.
The players were controlled by a neural network that controlled the setting strategy. Using our evolutionary design tool FREVO, we evolved the behavior in order to maximize the profit for each player. There was a pool of players controlled by neural networks. After several rounds, the more successful (thus richer) individuals were allowed to stay in the pool and produce more offspring than the less successful ones.
In the first scenario the payout was the pot times three. So if, everybody would cooperate, you can earn your money gets tripled. If the maximum bet was 20$ this means a 60$ return, in other words a 40$ revenue. But if everybody in a group pays in, it's even better to defect - let's say five out of six cooperate, you get a 50$ revenue.
The game was played iteratively 10 rounds. Originally, we expected a strategy like Tit-for-Tat to evolve and prevail. However, defection turned out to be the only stable strategy. For each system state, individuals with the defecting gene could make more revenue. In other words, ruthless behavior paid off.
The situation changed, when we introduced a "synergy factor" into the payoffs. This meant that the money of cooperating players was not multiplied linearly, but over proportionally. Assume you are working with some colleagues on a common project, let's say writing a book. If you alone invest enough time into you chapter, the book still sucks because of the other chapters which are lame or missing. If half of the authors cooperate, the book might be accepted by a mediocre publisher, but still would not be that promising. But if everybody cooperates, the result is not double the revenue of the 50% case but much more!
In the experiment we reflected this issue by a quadratic factor in the pot function. Evolving the stable strategies again showed that after some generations of defecting players, cooperation evolved as a stable strategy!

This still gives hope for our civilization - although reading the daily newspaper does not always feed this hope.

No comments:

Post a Comment