Repeated games with partner choice

Christopher Graser, Takako Fujiwara-Greve, Julián García and Matthijs van Veelen

PLOS Computational Biology, 2025, vol. 21, issue 2, 1-19

Abstract: Repetition is a classic mechanism for the evolution of cooperation. The standard way to study repeated games is to assume that there is an exogenous probability with which every interaction is repeated. If it is sufficiently likely that interactions are repeated, then reciprocity and cooperation can evolve together in repeated prisoner’s dilemmas. Who individuals interact with can however also be under their control, or at least to some degree. If we change the standard model so that it allows for individuals to terminate the interaction with their current partner, and find someone else to play their prisoner’s dilemmas with, then this limits the effectiveness of disciplining each other within the partnership, as one can always leave to escape punishment. The option to leave can however also be used to get away from someone who is not cooperating, which also has a disciplining effect. We find that the net effect of introducing the option to leave on cooperation is positive; with the option to leave, the average amount of cooperation that evolves in simulations is substantially higher than without. One of the reasons for this increase in cooperation is that partner choice creates endogenous phenotypic assortment. Compared to the standard models for the co-evolution of reciprocity and cooperation, and models of kin selection, our model thereby produces a better match with many forms of human cooperation in repeated settings. Individuals in our model end up interacting, not with random others that they cannot separate from, once matched, or with others that they are genetically related to, but with partners that they choose to stay with, and that are similarly dependable not to play defect as they are themselves.Author summary: The two mechanisms studied most in the literature on the evolution of cooperation are population structure (or kin selection), and repetition, which can allow for reciprocity to evolve. In the literature on repeated games, it is typically assumed that the matching is random and exogenous. However, not all human interactions in which there is scope for cooperation take place between individuals that have no say in who they play with, or between individuals that are genetically related. In many interactions, individuals can decide to stay with their partner, or leave and find someone else to play their repeated games with. We show that if we include the option to leave in an otherwise classical setting of repeated interactions, partner choice can evolve and maintain higher levels of cooperation than reciprocity does in the standard setting, where individuals cannot leave their partner. This points to the power of partner choice.

Date: 2025
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Persistent link: https://EconPapers.repec.org/RePEc:plo:pcbi00:1012810

DOI: 10.1371/journal.pcbi.1012810

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