Friend or foe: subjective expected relative similarity as a determinant of cooperation.

Subjective expected relative similarity (SERS) is a descriptive theory that explains cooperation levels in single-step prisoner's dilemma (PD) games. SERS predicts that individuals cooperate whenever their subjectively perceived similarity with their opponent exceeds a situational index, namely the game's similarity threshold. A thought experiment and 2 experimental studies illustrate and explore SERS's characteristics, showing that the theory predicts cooperation and competition in single-step PD games under 3 informational structures: (a) clear and transparent similarity cues, (b) experienced similarity, and (c) semantic similarity. The study's findings suggest that perceived similarity and its application in SERS play an important role in the evolution of cooperation underlying both kin and group selection mechanisms.