Robust Altruism by Similarity in Multifaceted Phenotypes*

We study the evolution of altruistic behavior under a model where individuals choose to cooperate with each other by comparing a set of always-visible continuous tags, and find the generic maintenance of robust cooperation when individuals carry more than one tag. Our model marries existing weak and strong selection results in the literature, addresses previous challenges to prior model formulations and supports the viability of a simple memory-less mechanism for cooperation in a well-mixed population. Robust cooperation is buoyed by a regime of weak positive selection that serves as a buffer for the strongly selected cycles of high cooperation.