Indirect reciprocity
Be good to others and a watching world will be good to you.
What it means
A mechanism for the evolution of cooperation in which helping someone earns a good reputation that prompts third parties to help you later, even if the original beneficiary never reciprocates. Unlike direct reciprocity, it sustains cooperation among people who may never interact twice, provided reputations are observed and transmitted through gossip and observation. Nowak and Sigmund showed it can be evolutionarily stable when the cost of reputation-tracking is low, and it depends on shared social norms about who deserves help. It matters because it explains large-scale human cooperation, the power of reputation systems, and why people behave better when they believe they are being watched.
Examples
A seller who treats every customer fairly earns online reviews that bring new buyers who never met the earlier ones.
An engineer who patiently answers beginners' questions on a public forum finds, years later, that strangers volunteer to review her own code — none of them people she ever helped.
A neighbour who clears everyone's snow gets talked about, and when he is later laid up ill, meals arrive from households he has never met.
First described in Alexander (1987); Nowak & Sigmund (1998, 2005).