Adaptive dynamics of direct reciprocity with N rounds of memory

TL;DR

This paper analyzes the adaptive dynamics of direct reciprocity strategies with N rounds of memory in repeated donation games, revealing symmetries and analytical properties of higher-memory strategies.

Contribution

It provides the first analytical results on the evolutionary dynamics of strategies with arbitrary memory length in repeated interactions.

Findings

Existence of a backward orbit for every forward orbit in the dynamics.

Decomposition of payoffs into symmetric and anti-symmetric parts.

Identification of symmetries when interchanging players and strategies.

Abstract

The theory of direct reciprocity explores how individuals cooperate when they interact repeatedly. In repeated interactions, individuals can condition their behaviour on what happened earlier. One prominent example of a conditional strategy is Tit-for-Tat, which prescribes to cooperate if and only if the co-player did so in the previous round. The evolutionary dynamics among such memory-1 strategies have been explored in quite some detail. However, obtaining analytical results on the dynamics of higher memory strategies becomes increasingly difficult, due to the rapidly growing size of the strategy space. Here, we derive such results for the adaptive dynamics in the donation game. In particular, we prove that for every orbit forward in time, there is an associated orbit backward in time that also solves the differential equation. Moreover, we analyse the dynamics by separating payoffs into a symmetric and an anti-symmetric part and demonstrate some properties of the anti-symmetric part. These results highlight some interesting symmetries that arise when interchanging player one with player two, and cooperation with defection.