The free-rider problem
Public Goods
You and a partner each have a private stake. You may keep it, or put some in a shared pot. The pot grows, then is split equally — including to whoever contributed nothing. The group is richer if everyone chips in. Each person is richer if they do not.
Public goods are the n-person cousin of the Prisoner's Dilemma. Anything you contribute is multiplied and shared, including with people who contributed nothing. The group wants full contribution. Each person wants to ride for free. When the marginal return on your own token is less than one, zero is a dominant strategy.
Voluntary contribution experiments are famous for a hump-then-decay: people start generous, then contributions slide toward the equilibrium unless something restores reciprocity — punishment, communication, or a partner who matches you.
This version is two players with a slider, so you can see the same logic without a crowd. Tit-for-Tat here means full contribution first, then copying whatever you just put in.
What theory predicts
Because the marginal return on your own contribution is less than 1, contributing nothing is a dominant strategy. The unique Nash equilibrium is both free-ride, even though both contributing everything pays more. It is a social dilemma, like the Prisoner's Dilemma with a slider.
What people actually do
People start by contributing a lot — often half or more — then contributions decay toward zero unless there is punishment, communication, or a partner who reciprocates. Conditional cooperation is the typical lab pattern.