A simulation study identifies robust social norms that sustain cooperation in costly reputations

A groundbreaking study led by Prof. Dr. Hitoshi Yamamoto (Rissho University) and his collaborators from, Soka University, Koriyama Women’s College, and the RINRI Institute has made significant strides in the field of indirect reciprocity: a key mechanism for sustaining cooperation in human societies. Their research unveils new insights into the social norms that protect cooperative behaviour from defector invasion and reputation costs, thereby deepening our understanding of how large-scale societies maintain stability. The results of the study were published in Scientific Reports.

Indirect reciprocity is characterised by cooperative behaviour being returned by third parties rather than the original recipient, as is the case with direct reciprocity. While previous studies have largely focused on the costs of cooperation, this research shifts the spotlight to the costs associated with reputation management—a vital, yet underexplored, factor in cooperation dynamics.

Prof. Dr. Yamamoto and his team employed an agent-based simulation to explore the resilience of the "leading eight" social norms. These norms have been identified as fundamental to stabilizing cooperation in indirect reciprocity. Through evolutionary computation method, the team demonstrated that two norms stand out as particularly robust against defector invasions and reputation costs.

The study found that four of the leading eight norms exhibit strong resilience against defectors due to their neutral stance on justified defection. These norms do not penalize justified defection, allowing them to protect cooperative behaviour even in the face of defector invasions. Additionally, the research revealed that another set of norms resists reputation costs by negatively evaluating unjustified cooperation. This allows societies to exclude "free-riders" who cooperate without contributing to reputation-sharing efforts, ensuring that cooperation remains stable even under costly reputation systems.

The study highlights the importance of understanding the mechanisms behind reputation-sharing in large societies and encourages further exploration into the costs and benefits of these systems. By identifying norms that can withstand both defector invasion and reputation costs, the research opens the door to more resilient cooperative structures in both human societies. Prof. Dr. Yamamoto, the lead author, emphasizes: "Our findings challenge the conventional focus on direct costs of cooperation by demonstrating that reputation costs play a critical role in sustaining large-scale cooperation. This insight provides a new avenue for research into how societies can foster and maintain cooperative behaviour."

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Clarifying social norms which have robustness against reputation costs and defector invasion in indirect reciprocity

Hitoshi Yamamoto, Isamu Okada, Tatsuya Sasaki & Satoshi Uchida

Scientific Reports volume 14, Article number: 25073 (2024)

https://doi.org/10.1038/s41598-024-76168-5

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