Generalized (indirect) reciprocity
You help B, B helps C, and somehow it comes back to you. Reputation is the currency.
Grounded in the research on Indirect reciprocity (Wikipedia)
What it actually is
Direct reciprocity is you scratch my back, I scratch yours, same two people, over and over. Generalized (indirect) reciprocity is bigger: A helps B, B pays it forward to C, and the favor eventually loops back to A through people who never directly owed them. The Wikipedia framing: randomly paired encounters in a population where the same two people need not meet again. Blood banks and food banks run on exactly this. You give to a network, not a person, and trust the network keeps existing.
The reputation math
Robert Trivers showed direct reciprocity evolves when the chance you meet again (w) beats the cost-to-benefit ratio: w > c/b. Indirect reciprocity swaps that out for reputation. Cooperation survives when q > c/b, where q is the probability that others KNOW your reputation. Translation: helping is only worth it if word gets around. No gossip, no reputation tracking, no cooperation. The whole system runs on people knowing who the givers are.