Cooperation evolves readily if individuals can walk away from uncooperative partners


Much research tries to understand how animals including humans have evolved to be very cooperative despite the temptation to exploit others for selfish gain. An important tool of this research is game theory, especially the Prisoner’s dilemma game with multiple rounds in which players have to choose whether to cooperate or defect. The payoffs mean that it’s always tempting to defect on one round (see table). One way of getting cooperation is by getting into a long sequence of both cooperating (3 points), which beats a mixture of both partners cooperating and defecting (average 1.75). But an outstanding problem is how to stop cheats exploiting a group of cooperative individuals and causing everyone to give up cooperating.


This model assumes that individuals interact in a world where they move around between partners in games of Prisoner’s dilemma. This model was used to test whether a simple approach to playing the game (“strategy”) could win overall by staying with a partner that cooperates and move away if the partner defects. Many simulations showed that this “Walk Away” strategy wins most of the time, including beating strategies that have been previously shown to win in these types of game against other strategies. More simulations showed that Walk Away also beats strategies that try to invade once all individuals are playing Walk Away. The success of Walk Away did depend on how easy it was to find another partner, but does well in most situations.


Many clever strategies have been suggested for playing Prisoner’s dilemma, but often very simple strategies win. Two important ones are Tit for Tat which copies the partner’s last move, and Win Stay Lose Shift, which repeats the last move if the partner cooperates but changes if they defect. This study has shown that even these strategies can be beaten by a strategy that simply refuses to stay with an individual that defects, and this is because they still stay with partners who sometimes defect. This new strategy means that cooperators end up staying together and excluding defectors, and this may explain how cooperation can evolve in very simple animals, depending only on their ability to change their partner or neighbour.


Behavioural ecology

Subject Group

Zoology and Ecology




Prisoner's dilemma




Posted by


on Fri Jun 26 2020

Article ID


Details of original research article:

Aktipis CA. Know when to walk away: contingent movement and the evolution of cooperation. Journal of Theoretical Biology. 2004;231:249-260.

Preceded by:

Understanding the interactions between individual animals requires appreciating that they have flexible responses to each other, rather than have fixed behaviour

Posted by: AndrewDHigginson Posted Fri Oct 27 2017

Followed by:

Clever game players that extort their partner in the Prisoner’s dilemma do well head-to-head but do poorly in a mixed population of different players

Posted by: AndrewDHigginson Posted Mon Jul 16 2018

A clever game player can choose a strategy than ensures it beats an opponent in the Prisoner’s dilemma game

Posted by: AndrewDHigginson Posted Mon Jul 16 2018


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