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
Individual
• 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
Larger
Punishments
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
Treatment,
Cooperation
Increases as
Defectors Switch to
Cooperation
3. Within No
Punishment,
Cooperation Declines
as More Individuals
Defect
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
Cooperation”
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
Preferred
After Period 5, SI Strongly
Preferred
2. Average Contribution to
Public Good
(Symbols)
SFI Declines to 0
SI Increases to > 90%
Gurerk et al. (2006) Science
312:108.
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
Punishment
Combined with
Reputation Game