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. 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 PresentationTRANSCRIPT
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 Seems to create trust from thin air Anecdotal evidence suggests reputation
systems work pretty well
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
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?
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
Interaction game Simultaneous, perfect
information Played repeatedly
until a pair willing to interact is found
No direct payoffs Small penalty for
failing too often
Transaction Game Simultaneous, perfect
information Agents choose
whether to cooperate or defect
Payoffs based on both agents’ behavior
Instance of the Prisoners’ Dilemma
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
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
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
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
Unconstrained Evolution
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
Disabled Retaliation
Results improve further if we force the 1st negative rate to 1.0
Retaliation clearly is an obstacle to cooperation
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
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
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
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