Prevolio
Game theoryClassroom · multi-user

Public Goods Game — Save the Commons

A class-wide cooperation game: each round, contribute to a shared Climate Fund and watch how free-riding emerges.

You’ll learn: Understand why shared goods get under-funded and how individual incentives drive free-riding.

Teacher guide

Students will learn to

  • See why a shared resource everyone benefits from tends to be under-funded.
  • Feel the tension between the group's best outcome and each person's private incentive.
  • Recognize free-riding and why cooperation often erodes over repeated rounds.

Before the session — ask

  • If everyone chips in, the whole group is better off — so why might someone hold back?
  • Where in real life do we rely on people voluntarily contributing to a shared good?

After the session — discuss

  • Did average contributions rise, hold steady, or fall over the rounds? Why?
  • What was the gap between what the class earned and the best possible group outcome?
  • What rules or norms might keep people contributing — taxes, reputation, punishment?

Timing

  • Briefing & the rules3–5 min
  • Eight contribution rounds8–12 min
  • Debrief & discussion8–10 min

Run it with your class

Students join with a code or QR — no account needed. Teachers start a session from the dashboard.

Join with a codeFor teachers

What is the public goods game?

The public goods game shows why things that benefit everyone tend to be under-funded. Each player privately decides how much of their own money to contribute to a shared pot; the pot is then multiplied and split equally among all players, whether or not they contributed. Because the return on your own contribution is shared with everyone, each individual is tempted to contribute little and enjoy the benefits paid for by others — that is free-riding. If everyone reasons this way, the group contributes far less than would be best for all, and the public good is under-provided even though everyone would gain from full cooperation. This is the core of the collective action problem: a public good is non-excludable (you can't stop non-contributors from enjoying it), so private incentives pull against the common interest. It is the mechanism behind under-funded shared resources everywhere — clean air, public broadcasting, vaccination, climate action. Real groups often start out cooperating but see contributions decay as free-riding spreads. The lesson is why voluntary contribution alone rarely sustains a public good, and why institutions — taxes, rules, reputation, and punishment of free-riders — exist to hold cooperation together.

How the simulation shows it

Each round the class contributes to a shared Climate Fund that's multiplied and split among everyone, and you watch cooperation erode as free-riding spreads. Choosing your own contribution while others hold back makes the tension concrete: the group does best if all give, but each person is individually tempted to give less.

Common misconceptions

Frequently asked questions

What is a public good?
A good that is non-excludable and shared: you can't easily stop non-contributors from enjoying it, and one person's use doesn't use it up. Clean air and public broadcasting are examples.
What is free-riding?
Enjoying the benefit of a shared good without paying your share. Because the return on a contribution is split among everyone, each person is tempted to contribute little and rely on others.
Why are public goods under-funded?
Because private incentives pull against the common interest: each individual gains more by contributing less and free-riding, so voluntary contributions fall short of what's best for the group.
What is the collective action problem?
The difficulty of getting people to cooperate for a shared benefit when each is individually better off not contributing. The public goods game is its classic illustration.
How can cooperation be sustained?
Through institutions that change incentives — taxes or mandatory contributions, rules, reputation, and the ability to punish free-riders — since voluntary giving alone usually decays over time.

Related concepts