Evolution of vulnerability to pain in interpersonal relations as a strategic trait aiding cooperation

Why are humans so vulnerable to pain in interpersonal relations and can so easily hurt others physically and emotionally? We theoretically examine whether being offensively strong but defensively weak can evolve as a strategic trait that fosters cooperation. We study a population comprised of “thick-skinned” and “thinskinned” agents by using an indirect evolution model that combines rational choice in strategic interactions with evolutionary selection across generations. We find that (a) the relatively vulnerable and cooperative thin-skins cannot evolve under purely random matching, (b) with some assortment thin-skins evolve and can take over the entire population, (c) vulnerability to greater pain makes it easier for thin-skins to evolve, and (d) proximate pain which merely feels bad but does not lower fitness helps thin-skins evolve even more than pain which accurately reflects fitness consequences. We draw contrast with the Hawk-Dove model and identify several ways in which rationality hinders the evolution of the relatively vulnerable and peaceful type of agent.