A strategy of win-stay, lose-shift that outperforms tit-for-tat in the Prisoner's Dilemma game

The simulations display occasional bursts of defection, which are based on a previous build-up of undiscriminating altruists.

“The simulations display occasional bursts of defection, which are based on a previous build-up of undiscriminating altruists.”

In a population of discriminators, unconditional cooperators can increase by random drift and eventually invite the invasion of defectors.

“In a population of discriminators, unconditional cooperators can increase by random drift and eventually invite the invasion of defectors.”

The cooperative outcome in the indirect reciprocity model lacks stability.

“The second issue concerns the lack of stability of the cooperative outcome.”

Individual-based simulations show that if the cost-to-benefit ratio is sufficiently low, and the amount of information about the co-player's past sufficiently high, cooperation based on discrimination can emerge.

“Individual-based simulations show that if the cost-to-benefit ratio is sufficiently low, and the amount of information about the co-player's past sufficiently high, cooperation based on discrimination can emerge.”

In the resulting population, help is channelled towards those who help.

“In the resulting population, help is channelled towards those who help 35-37 .”

A more sophisticated assessment rule should distinguish between justified and unjustified defection, taking into account the score of the receiver.

“A more sophisticated assessment rule should distinguish between justified and unjustified defection, and thus take into account the score of the receiver: someone withholding help from a 'bad' player should not pay with a reduced score 35,38,39,47 .”

The concept of justified defection requires information about the past of the co-player, the co-player's co-players, and their co-players etc.

“The problem with the concept of justified defection, however, is that it requires information, not only about the past of the co-player, but also the past of the co-player's co-players, and their co-players etc 35,39 .”

Private lists of the scores of co-players are very plausible if individual interactions are only observed by a fraction of the population.

“Such private lists of the scores of co-players are very plausible if individual interactions are only observed by a fraction of the population 35 .”

Observer Tit For Tat relies on reputation in the first round and on personal experience in all further rounds against the same co-player.

“Observer Tit For Tat relies on reputation in the first round and on personal experience in all further rounds against the same co-player.”

A good strategy for the latter (interested audiences) is Observer Tit For Tat.

“A good strategy for the latter is Observer Tit For Tat 35,68 .”

Often there is evidence for a dual motivation: players give donations if the recipient's score is high or their own score is low.

“Often there is evidence for a dual motivation players give donations if the recipient's score is high or their own score is low 35 .”

For indirect reciprocity, a similar rule holds: the probability q to know the social score of another person must exceed the cost-to-benefit ratio.

“For indirect reciprocity a similar rule holds 35 : the probability, q , to know the social score of another person must exceed the cost to benefit ratio”

In a natural extension of the basic model of indirect reciprocity, an action between a donor and a recipient is observed by a subset of the population.

“In a natural extension of the basic model of indirect reciprocity, an action between a donor, A, and a recipient, B, is observed by a subset of the population 35 .”