The evolution of theory of mind on welfare tradeoff ratios

People seem to attribute beliefs and desires to another person when interacting with them. Such a “theory of mind” capacity is essential for complex and uniquely human behavior such as language, but its evolutionary origin remains elusive. Using the formal tools of evolutionary game theory, we asked what environmental properties are necessary to select for a basic form of theory of mind—the ability to infer the prosociality, quantified by the welfare tradeoff ratio, of another person toward oneself. We found that none of the environments studied in classical or evolutionary game theory give an advantage to this form of theory of mind capacities; theory of mind is advantageous only in a new class of environments with stable opponents and variable payoff structures. In two behavioral experiments (n = 91) we verified that people can, and do use theory of mind in such an environment. These results suggest that some features of early humans’ social environment that were previously neglected in evolutionary game theory may be responsible for the evolution of people’s complex social capacities.