# A norm knockout method on indirect reciprocity to reveal indispensable norms

## TL;DR

This paper introduces a norm knockout method inspired by genetic engineering to identify essential norms for cooperation in indirect reciprocity, revealing the transition to tolerant norms and the critical role of intolerant norms.

## Contribution

The study develops a novel norm knockout approach to determine indispensable norms and elucidates the co-evolutionary dynamics of norms and cooperation.

## Key findings

- Most norms shift to tolerant norms after defectors are eliminated.
- Cooperation fails to emerge when intolerant norms are knocked out.
- Indispensable norms are identified as crucial for sustaining cooperation.

## Abstract

Although various norms for reciprocity-based cooperation have been suggested that are evolutionarily stable against invasion from free riders, the process of alternation of norms and the role of diversified norms remain unclear in the evolution of cooperation. We clarify the co-evolutionary dynamics of norms and cooperation in indirect reciprocity and also identify the indispensable norms for the evolution of cooperation. Inspired by the gene knockout method, a genetic engineering technique, we developed the norm knockout method and clarified the norms necessary for the establishment of cooperation. The results of numerical investigations revealed that the majority of norms gradually transitioned to tolerant norms after defectors are eliminated by strict norms. Furthermore, no cooperation emerges when specific norms that are intolerant to defectors are knocked out.