Public Goods Games

Public Goods & Reputation: Humans
Recall Eusocial Insects: Altruism
Inclusive Fitness  Kin Selection
Monogamous Queens
Multiply-Mated Queens  Lower r
Kinship, Coercion & Policing
Public Goods & Reputation: Humans
Cooperation Among Non-Relatives
Repeated Interaction
Conditional Cooperation, Reciprocity
Sufficient for Humans?
Degree of Cooperation: Maintained?
Punish Defectors? Coerce Cooperation?
Public Goods & Reputation: Humans
Experimental Approaches:
Economics & Behavioral Ecology
Public Goods Game
Indirect Reciprocity
Reputation Game
1. Public Goods Game: Example
4 players
• Each player: Endowment of 20 MU
• Contributes 0 to 20 MU to Group Project/Keeps Rest
• Reward All Players: 0.4(Total Contribution)
Public Goods Game: Example
“Free Rider” Does Not Invest
Takes Share of Public Good
Temptation to Defect
All 4 Defect; Each Retains 20 MU
All 4 Invest Fully: Each Earns 80(0.4) = 32 MU
Metaphor: Dilemma of Social Cooperation
2. Indirect Reciprocity Game: Example
Similar to Reciprocal Altruism
Donor Gives 0 - 3 MU to Specified Receiver
Public Donation  Reputation
Donor Later Becomes Receiver:
Third Individual as Donor
Reputation Earns Donation > 3 MU
If Donors Common, Stable Solution
How Get Common?
Fehr & Gachter (2002) Nature 415:137.
Human Evolution: Dangerous Hunts, Conserve
Resources, Warfare
Why Do Humans Invest in Costly Activity?
Kin Selection, Reciprocity
Costly Signaling to Gain Reputation as Donor
Humans Cooperate: Non-Kin, Mixed
Populations, Small Gain via Reputation
Fehr & Gachter (2002) Nature 415:137.
Punishment of Free Riders
Altruistic Punishment: Has Cost
Make Cooperation Pay?
Will Individual Pay to Punish Free Rider?
Entire Group Benefits
Incentive to Individual Altruist (Punish)
Fehr & Gachter (2002) Nature 415:137.
Public Goods Game with Punishment
240 Students; Groups with 4 Members
Two Treatments: Punishment & No Punishment
• Choose Public Goods Investment
• See All (Anonymous) Players Investment
• (Choose to Punish Other Players: If Available)
Fehr & Gachter (2002) Nature 415:137.
Public Goods: Invest 0 to Endowment
Punish: 0 to 10 MU (Cost to “Altruist”)
Punished Individual: Loses 3  Cost of Punisher
Spite? Altruism toward Other Investors
Hypothesis: Free Rider Later Cooperates
Group Composition Changed Frequently
1. Larger Degree of
Defection Incurs
2. Strength of
Punishment Stable
Across Time
3. Frequencies?
Fehr & Gachter (2002) Nature 415:137.
Fehr & Gachter (2002)
Nature 415:137.
1. Punishment Increases
Cooperation in Public
Goods Game,
Compared to Game
with No Punishment
2. Within Punishment
Increases as
Defectors Switch to
3. Within No
Cooperation Declines
as More Individuals
Fehr & Gachter (2002) Nature 415:137.
Suggest: Free Riders (Defectors) Learn to Expect
Costly Punishment and Begin to Cooperate
Suggest: Free Rider Triggers Anger, Larger
Difference in Contribution Reports More Anger
Gurerk et al. (2006) Science 312:108.
Sanctioning (Punishment) “Stabilizes Human
Observe Lack of Sanctioning Lead to Decay of Cooperation
Stable Cooperation: Enough Individuals Willing to Pay Cost of
Punishing Defectors
Identify Free-Riders and Cooperation Common: Stable
Gurerk et al. (2006) Science 312:108.
Will Individuals Prefer to Join Sanctioning
Institution If Sanction-Free Institution Available?
Experiment: 84 Subjects, Game in Stages
• S0: Choose Sanctions (SI) or Sanction Free (SFI)
• S1: Contribution to Public Goods Game
(Social Dilemma)
• S2: Sanctioning (If Available)
1. Subjects’ Choices (Bars)
First 2 Periods, SFI
After Period 5, SI Strongly
2. Average Contribution to
Public Good
SFI Declines to 0
SI Increases to > 90%
Gurerk et al. (2006) Science
Payoffs Approach Maximum in Sanctioning
Gurerk et al. (2006)
Science 312:108.
Gurerk et al. (2006) Science 312:108
• Initial Reluctance, SI Rapidly Dominant Choice
Cooperation Becomes Common ( Stable)
Cooperators, with Punishing, Earn More
• Free-Riders in SI Earn Less, Due to Punishment
• Within SFI, Cooperation Collapses
Rockenbach & Milinski (2006) Nature 444: 718.
Interaction: Indirect Reciprocity & Costly Punishment
Humans: Show Altruism, But Misuse Public Goods
Overfish Oceans, Risk Climate-Change Disaster
Through Increasing Use of Fossil Fuels, …
“Tragedy of Commons”
Why Cooperate?
Rockenbach & Milinski (2006) Nature 444: 718.
Costly (“Altruistic”) Punishment of Defectors
Maintain Reputation (Indirect Reciprocity)
Assert: If Indirect Reciprocity Effective, Fewer
Defectors to Punish (Cost Reduced)
Rockenbach & Milinski (2006) Nature 444: 718.
Groups of 8 Subjects Played Public Goods Game
for 20 Periods
2 Treatments (Each Present/Absent)
Choose Punishment or Punishment-Free
Both Play Public Goods Game
Punishment Stage (If Applicable)
Play Indirect Reciprocity Game (If Applicable)
Rockenbach & Milinski (2006) Nature 444: 718.
Punishment AND Indirect Reciprocity
(a) Largest PG Contribution, Most Cooperative
(c) By Period 3, Most Common Choice
Rockenbach & Milinski (2006) Nature 444: 718.
Cost of Punishment
Reduced IF
Combined with
Reputation Game

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