Yamamoto, H.(山本仁志), Suzuki, T.(鈴木貴久), & Umetani, R.(梅谷凌平) (2020).
Justified defection is neither justified nor unjustified in indirect reciprocity.
PLOS ONE, 15(6), e0235137.
doi: 10.1371/journal.pone.0235137

Indirect reciprocity is one of the major mechanisms in the evolution of human cooperation. In indirect reciprocity, social norms with which individuals distinguish good people from bad people play essential roles. Despite extensive studies on the evolution of cooperation in indirect reciprocity, little is known about which social norms people actually adopt. Here we reveal what kind of norms are adopted by people in indirect reciprocal situations in daily life by using scenario-based experiments. The results showed that people evaluated “justified defection” as neither good nor bad and withheld their evaluation. Theoretically, social norms that evaluate justified defection as good are required for cooperation to be stable. However, the norm that people actually adopted deviates from the theoretical predictions. Our results indicate the necessity to reconsider the justification of “justified defection” in the evolution of cooperation.