The influence of language on the evolution of cooperation

Humans' sophisticated language system and advanced ability to cooperate with non-relatives are unique amongst animals. Verbal arguments have asserted that language, in particular, facilitated the evolution of large-scale cooperation—but the specific mechanisms by which language influences cooperation have not been tested in formal models. Here we develop a mathematical model that explicitly considers three possible influences of language on the evolution of cooperation: 1) increasing the payoff of cooperative interactions, 2) allowing cooperation to succeed with fewer cooperators in the group, and 3) promoting the positive assortment of cooperators. Our results show that the role of language in the evolution of cooperation is not as straightforward as often believed, with each model variant showing a different effect of language. Notably, when language decreases the number of cooperators needed to successfully reach a positive group payoff, the equilibrium frequency of cooperators can actually decrease with increasing language proficiency. Language does consistently favor the evolution of cooperation when it leads to higher payoffs of cooperation or facilitates the positive assortment of cooperators. However, language cannot lead to the evolution of cooperation in a population of only defectors, except when it leads to positive assortment in both cooperators and defectors. Overall, our results demonstrate that the way that language alters cooperative interactions determines its effect on social evolution.