Characterizing oscillations in heterogeneous populations of coordinators and anticoordinators

Abstract

Oscillations often take place in populations of decision makers that are either a coordinator, who takes action only if enough others do so, or an anticoordinator, who takes action only if few others do so. Populations consisting of exclusively one of these types are known to reach an equilibrium, where every individual is satisfied with her decision. Yet it remains unknown whether oscillations take place in a population consisting of both types, and if they do, what features they share. We study a well-mixed population of individuals, which are either a coordinator or anticoordinator, each associated with a possibly unique threshold and initialized with the strategy A or B. At each time, an agent becomes active to update her strategy based on her threshold: an active coordinator (resp. anticoordinator) updates her strategy to A (resp. B) if the portion of other agents who have chosen A exceeds (falls short of) her threshold, and updates to B (resp. A) otherwise. We define the state of the population dynamics as the distribution over the thresholds of those who have chosen A. We show that the population can admit several minimally positively invariant sets, where the solution trajectory oscillates. We explicitly characterize a class of positively invariant sets, prove their invariance, and provide a necessary and sufficient condition for their stability. Our results highlight the possibility of non-trivial, complex oscillations in the absence of noise and population structure and shed light on the reported oscillations in nature and human societies.