国际清算银行-完全去中心化系统中讲真话的一个不可能定理（英）-2023.8-17页_347kb

BIS Working Paper Summary

Title: An impossibility theorem on truth-telling in fully decentralized systems
Authors: Rodney Garratt, Cyril Monnet
Date: August 2023

Introduction and Abstract

• The paper demonstrates that truthful reporting of a publicly observed event cannot be achieved as a unique equilibrium in a fully decentralized environment.
• This impossibility arises due to incentive issues in decentralized voting systems, where agents manipulate outcomes for personal gain.
• The findings provide a theoretical foundation for the need of oracles and address the oracle problem in decentralized systems like smart contracts.

Key Arguments and Findings

• Simultaneous Voting Example:

  • In a coin toss scenario with multiple agents, truthful voting is only a dominant strategy if rewards solely depend on matching the outcome. Otherwise, equilibria emerge where all agents vote according to the majority bet, regardless of the true state.
  • If rewards for truthful voting are split (γ < 1), agents become indifferent to truth, leading to multiple equilibria where manipulation prevails.

• Sequential Voting Analysis:

  • Sequential voting does not resolve the truth-telling issue; agents still coordinate on a majority-based outcome if it aligns with their incentives.
  • A unique perfect Bayesian equilibrium exists only when majority voting is forced, but this outcome is not truthful and depends on initial bets rather than the true state.

• Impossibility Theorem:

  • In a large, decentralized system, no mechanism can uniquely implement truthful reporting as a Bayesian Nash equilibrium.
  • Agents must be indifferent to the truth for truthful behavior to occur, which is impractical without external incentives.
  • This highlights limitations in peer prediction games (PPG), which differ in having no common knowledge and private signals.

Conclusion

• Fully decentralized systems cannot function effectively for truth-telling without some form of trusted authority or incentive.
• The oracle problem persists because individuals may free-ride on others' truthful reporting.
• The paper underscores the need for further research on oracle mechanisms to decentralize truth verification.