when a good reputation isn’t good enough

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When a Good Reputation isn’t Good Enough Jonathan Traupman Robert Wilensky U.C. Berkeley

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When a Good Reputation isn’t Good Enough. Jonathan Traupman Robert Wilensky U.C. Berkeley. Introduction. Reputation systems are a key component of many peer-to-peer systems Lots of application specific features, but most share a common structure Aggregates feedback about transactions - PowerPoint PPT Presentation

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Page 1: When a Good Reputation isn’t Good Enough

When a Good Reputation isn’t Good Enough

Jonathan Traupman

Robert Wilensky

U.C. Berkeley

Page 2: When a Good Reputation isn’t Good Enough

Introduction Reputation systems are a key component

of many peer-to-peer systems Lots of application specific features, but

most share a common structure Aggregates feedback about transactions Seems to create trust from thin air Anecdotal evidence suggests reputation

systems work pretty well

Page 3: When a Good Reputation isn’t Good Enough

Evaluation is difficult

E.g. peer-to-peer markets Many trust signals besides reputation Market and payment provider offer some

indemnity Difficult to separate out the role of the

reputation system Cannot realistically experiment with

alternative reputation systems

Page 4: When a Good Reputation isn’t Good Enough

Some Questions In the absence of external forces, is a

reputation system sufficient for encouraging cooperation?

Under what conditions does stable cooperation arise?

How well or poorly do existing reputation systems meet these requirements?

How can we design systems to better encourage cooperation?

Page 5: When a Good Reputation isn’t Good Enough

A Game-theoretic model Model the trading process as a series of

simple games1. Interaction game: agents decide whether or not to

trade2. Transaction game: agents decide to cooperate or

defect3. Reputation game: agents decide how to leave

feedback Observe what strategies are optimal under

different conditions

Page 6: When a Good Reputation isn’t Good Enough

Interaction game Simultaneous, perfect

information Played repeatedly

until a pair willing to interact is found

No direct payoffs Small penalty for

failing too often

Page 7: When a Good Reputation isn’t Good Enough

Transaction Game Simultaneous, perfect

information Agents choose

whether to cooperate or defect

Payoffs based on both agents’ behavior

Instance of the Prisoners’ Dilemma

Page 8: When a Good Reputation isn’t Good Enough

Reputation Game Mixed game

Perfect information First move simultaneous Subsequent moves

sequential No direct payoff, but

outcome influences reputation

“Tragedy of the Commons” Honest reputations benefit

the community Individuals benefit from

dishonesty or apathy

Page 9: When a Good Reputation isn’t Good Enough

Evolutionary Simulator Repeatedly play the

three games Periodically evaluate

agent performance Mean payoff per

transaction

Keep successful agents “Breed” new agents by

combining parameters of successful parents

Interaction parameters New user interactivity Low-exp. interactivity High-exp. Interactivity

Transaction parameters Honesty

Reputation parameter 1st negative rate 1st positive rate Retaliation rate

Page 10: When a Good Reputation isn’t Good Enough

Experiments All experiments performed 20 times1. Unconstrained evolution

Reputation system modeled after Percent Positive Feedback (PPF) on eBay

2. Retaliation prohibited Retaliation rate parameter forced to zero

3. Simultaneous feedback Retaliation rendered impossible by forcing agents

to leave feedback blind

Page 11: When a Good Reputation isn’t Good Enough

Unconstrained Evolution Most similar to current market conditions None of the markets were able to maintain

cooperation Moderate (~50%) retaliation rate Retaliation caused all agents to hesitate leaving

feedback first Dysfunctional reputation system permits

defection to emerge as the optimal strategy

Page 12: When a Good Reputation isn’t Good Enough

Unconstrained Evolution

Page 13: When a Good Reputation isn’t Good Enough

Disabled Reputation Knock out the retaliation parameter 14 of 20 markets remained cooperative for

10,000 generations 2 oscillated 4 remained uncooperative Much higher participation in the reputation

system Lack of direct incentives for honest feedback

allowed agent apathy to prevent cooperation

Page 14: When a Good Reputation isn’t Good Enough

Disabled Retaliation

Results improve further if we force the 1st negative rate to 1.0

Retaliation clearly is an obstacle to cooperation

Page 15: When a Good Reputation isn’t Good Enough

Simultaneous Feedback Disabling retaliation outright is not possible in a

real marketplace Simultaneous feedback is a common

suggestion Modify the reputation game to be simultaneous

rather than sequential Can’t retaliate if you don’t know the feedback

you’re getting Still other ways to game the system, but a good

first step

Page 16: When a Good Reputation isn’t Good Enough

Simultaneous Feedback 19 of 20 markets oscillated One remained non-

cooperative throughout Agent apathy remains a

problem In highly cooperative

markets, agents get lazy about leaving, using feedback

Permits defectors to gain a foothold

Eventually, cooperation is restored

Page 17: When a Good Reputation isn’t Good Enough

Conclusions Under the right conditions, a simple reputation

system like PPF can maintain cooperation Users must participate frequently and honestly

As currently implemented, PPF cannot maintain cooperation on its own Permits retaliation Does nothing to prevent apathy

Basically confirms intuitions about reputation systems Provides a better theoretical and experimental

foundation for these arguments

Page 18: When a Good Reputation isn’t Good Enough

Conclusions

Provides some guidelines for designing better reputation systems Must prohibit retaliation and other means of

gaming the reputation system Should create incentives for honest

participation to combat user apathy