Altruism Improves Congestion in Series-Parallel Nonatomic Congestion Games

Self-interested routing policies from individual users in a system can collectively lead to poor aggregate congestion in routing networks. The introduction of altruistic agents (referred to as altruists), whose goal is to minimize other agents’ routing time in addition to their own, can seemingly improve aggregate congestion. However, in some network routing problems, it is known that altruists can actually worsen congestion compared to that which would arise if all agents had simply behaved selfishly. This paper provides a thorough investigation into the necessary conditions for altruists to be guaranteed to improve total congestion. In particular, we study the class of series-parallel nonatomic congestion games, where one sub-population is selfish and the other is altruistic. We find that a game is guaranteed to have improved congestion in the presence of altruists (regardless of their population size) compared to if all agents route selfishly, provided the path set for the network is symmetric (all agents can access all paths), and the path set cannot exhibit Braess’s paradox (a phenomenon we refer to as a Braess-resistant path set). Our results appear to be the most complete characterization of when behavior that is designed to improve total congestion (which we refer to as altruism) is guaranteed to do so.