Stationary analysis of adaptive aspiration dynamics in graph-structured games

Abstract

Aspiration dynamics can favor cooperation on finite graphs through a purely local satisfaction mechanism. In the model studied here, individuals compare their payoffs with adaptive aspiration levels and revise their strategies through a self-switching Fermi rule. Although the aspirations are continuous and endogenous, the resulting finite-population process has a unique stationary distribution for every finite selection intensity and every finite connected graph under degree-normalized average payoffs when restricted to the compact forward-invariant state space in which aspirations lie in the payoff interval. In the weak-selection regime, the first-order stationary abundance shift is positive when \(R+S>T+P\), negative when \(R+S T+P\) means that, under neutrality, the expected payoff of a cooperator exceeds that of a defector. The stationary variance reveals a different effect whose first-order correction is governed by the payoff-range contrast \((R-S)-(T-P)\). This contrast compares how strongly cooperator and defector payoffs respond to changes in local neighborhood composition and therefore measures the relative sensitivity of their payoffs to the local environment. Under degree-normalized average payoffs, these first-order results hold on every finite connected graph and are independent of degree sequence, topology, and aspiration learning rate. Simulations on regular and heterogeneous graphs compare averaged and accumulated payoffs and illustrate that graph heterogeneity and the aspiration learning rate can affect stationary behavior away from weak selection. The first-order coefficients are derived from the finite stochastic process and do not use pair approximation, mean-field closure, or a large-population limit.