, Volume 169, Issue 2, pp 301–333

Keep ‘hoping’ for rationality: a solution to the backward induction paradox

  • Alexandru Baltag
  • Sonja Smets
  • Jonathan Alexander Zvesper
Open Access

DOI: 10.1007/s11229-009-9559-z

Cite this article as:
Baltag, A., Smets, S. & Zvesper, J.A. Synthese (2009) 169: 301. doi:10.1007/s11229-009-9559-z


We formalise a notion of dynamic rationality in terms of a logic of conditional beliefs on (doxastic) plausibility models. Similarly to other epistemic statements (e.g. negations of Moore sentences and of Muddy Children announcements), dynamic rationality changes its meaning after every act of learning, and it may become true after players learn it is false. Applying this to extensive games, we “simulate” the play of a game as a succession of dynamic updates of the original plausibility model: the epistemic situation when a given node is reached can be thought of as the result of a joint act of learning (via public announcements) that the node is reached. We then use the notion of “stable belief”, i.e. belief that is preserved during the play of the game, in order to give an epistemic condition for backward induction: rationality and common knowledge of stable belief in rationality. This condition is weaker than Aumann’s and compatible with the implicit assumptions (the “epistemic openness of the future”) underlying Stalnaker’s criticism of Aumann’s proof. The “dynamic” nature of our concept of rationality explains why our condition avoids the apparent circularity of the “backward induction paradox”: it is consistent to (continue to) believe in a player’s rationality after updating with his irrationality.


Backward induction Dynamic logic Epistemic logic Public announcements Rationality 
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Copyright information

© The Author(s) 2009

Authors and Affiliations

  • Alexandru Baltag
    • 1
  • Sonja Smets
    • 2
    • 3
  • Jonathan Alexander Zvesper
    • 4
  1. 1.Oxford University Computing LaboratoryUniversity of OxfordOxfordUK
  2. 2.University of GroningenGroningenThe Netherlands
  3. 3.Oxford UniversityOxfordUK
  4. 4.Institute for Logic, Language and ComputationUniversiteit van AmsterdamAmsterdamNetherlands

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