C. Athena Aktipis

Contingent Movement Rules And the Evolution of Cooperation In Groups

In this model, agents have two parameters, I, the level of investment in the group fund, and T, the threshold for leaving the group.  When agents contribute to the group, their investment is multiplied by M, the multiplier, and then split evenly among all group members, resulting in a classic social dilemma.  Agent's decision rules specify that as long as the return is greater than T, they stay in the current group, but once the return has gone below T, agents leave, often 'budding off' to form new groups. 

Average investment level for the whole population can be seen in the graph at the bottom right, average level of cooperation within each group is shown with the following spectrum, where darker blue represents higher average investment:

  0    1    2    3    4    5    6    7  8    9  10 =>11

While the level of cooperation within each group decreases, the level of cooperation in the overall population can continue increasing, due to the fact that more cooperative groups grow more quickly, and less cooperative groups decrease in size.  To see this phenomenon in action, click on the link below:

Contingent Movement and the Evolution of Cooperation in Groups