Published: 23rd June 2016 DOI: 10.4204/EPTCS.215 ISSN: 2075-2180 |
It has been my pleasure to be part of the TARK community since the first conference at Asilomar, California, in 1986, principally due to the encouragement of Rohit Parikh, one of the founders of TARK. This conference is uniquely situated as one that brings together researchers from a wide variety of fields, including Artificial Intelligence, Cryptography, Distributed Computing, Economics and Game Theory, Linguistics, Philosophy, and Psychology. It has played an important role in our understanding of interdisciplinary issues involving reasoning about rationality and knowledge.
TARK 2015 had 63 submissions out of which 18 were accepted as contributed talks and 9 as poster presentations for the programme. I am very grateful for having the cooperation and advice of 17 other members of the multidisciplinary program committee: Eleonora Cresto (CONICET, University of Buenos Aires, Argentina), Clare Dixon (University of Liverpool, UK), Edith Elkind (Oxford University, UK), Amanda Friedenberg (Arizona State University, USA), Sujata Ghosh (Indian Statistical Institute, India), Andreas Herzig (IRIT, Toulouse, France), Bettina Klaus (University of Lausanne, Switerland), Kevin Kelly (Carnegie Mellon University, USA), Yoram Moses (Technion, Tel Aviv, Israel), Andrés Perea (Maastricht University, The Netherlands), Sophie Pinchinat (IRISA, Rennes, France), Francesca Rossi (University of Padova, Italy), Olivier Roy (Bayreuth University, Germany), Burkhard Schipper (University of California at Davis, USA), Hans van Ditmarsch (LORIA, France), Yanjing Wang (Peking University, China) and Michael Wooldridge (Oxford, UK). I thank them for their hard work in providing careful reviews and for the detailed discussions about the submissions. When papers are read across disciplines, there can be keen differences in what is considered good and important; I thank the committee members for trying their best to listen to other viewpoints.
We had four eminent invited speakers in this edition of TARK: Robin Clark of the University of Pennsylvania, USA, bringing a viewpoint from Psychology and Cognition to epistemic reasoning; Simon Huttegger, of the University of California, Irvine, USA, a philosopher's look at observational process and inductive logic; Sarit Kraus of Bar-Ilan University, Israel, on desiging computational agents for interacting with people, based on insights from game theory and logic; Marciano Siniscalchi, Northwestern University, USA, on foundations of rationality in sequential games. In addition, we had a tutorial on causal inference and causal discovery, jointly by Peter Spirtes of Carnegie Mellon University, USA, and Kun Zhang of Max Planck Institute for Intelligent Systems, Tübingen, Germany. I thank these speakers for insightful presentations, some of which have been included as abstracts / papers here.
The organizing team at Carnegie Mellon University, led by Kevin Kelly, did an excellent job of running the conference, and Joe Halpern provided advice and inspiration when needed, I thank them all.
Though the conference proceedings were prepared at the time of the meeting
and made available from the conference page (http://www.imsc.res.in/tark/tark15.html),
it could not be indexed by DBLP and other scholarly services. Hence the need
for a Post-Proceedings that could be indexed and accessed in the Web. EPTCS
kindly agreed to provide a venue for this, and I sincerely thank Krzysztof Apt
for suggesting this avenue, and Rob van Glabbeek for patiently guiding
this process. I thank Anantha Padmanabha, my colleague at IMSc, for tremendous
help with preparing this volume.
R. Ramanujam Institute of Mathematical Sciences, Chennai, India Programme Chair, TARK 2015 |
Sequential rationality is the prevalent notion of best response for dynamic games; it is an essential part of the definition of sequential equilibrium [9], perfect Bayesian equilibrium [6], and extensive-form rationalizability [11]. Abstracting from notational and other minor differences, a strategy si of player i is
sequentially rational if, beginning at any information set where i moves, si specifies a sequence of actions that is optimal given the beliefs that i holds at that information set about the play of her opponents.
While this notion is central to the theory of dynamic games, it raises both practical and methodological concerns. From a practical standpoint, it is not obvious how to reliably ascertain which strategy a player follows in a given dynamic game, and a fortiori whether it is or isn't sequentially rational. Consider a three-stage Centipede game [13], played by Ann (who moves at the first and third nodes) and Bob. Suppose that, as predicted by backward induction, Ann ends the game at the first node by moving Down. Then, we are unable to observe Bob's intended choice at the second node. Furthermore, to determine whether such a choice would have been optimal, had Ann chosen Across instead, we need to consider Bob's beliefs conditional upon an event that Bob does not expect to occur-a zero-probability event. While formally we can represent such beliefs, it is not clear how an experimenter might elicit them in practice.
Reinhard Selten's strategy method [16] is a widely used experimental procedure that is intended to elicit players' intended strategy choices in dynamic games. There is evidence that the strategy method is an effective elicitation procedure: see, e.g., [3]. But this finding actually raises further questions. The strategy method essentially asks players to simultaneously commit to a strategy, which is then implemented by the experimenter without possibility of subsequent intervention by the subjects. This reduces the original dynamic game to one in which players face non-trivial moves only in the initial stage; furthermore, such moves are simultaneous. Standard solution concepts such as sequential or perfect Bayesian equilibrium predict that players will maximize ex-ante expected payoffs when the strategy method is employed; therefore, such solution concepts do allow subjects to commit to strategies that are not sequentially rational in the original game. Refinements that incorporate the notion of invariance [8] do imply that subjects will only commit to strategies that are sequentially rational in the original game. However, there is ample experimental evidence that contradicts the invariance hypothesis [5, 15, 4, 7]. Thus, the received theory cannot at the same time explain the effectiveness of the strategy method, and account for violations of the invariance hypothesis.
These practical issues hint at a deeper methodological concern. Economics has long embraced the revealed-preference approach: assumptions about agents' tastes and beliefs should be testable, or elicitable, on the basis of observable choices in suitably designed problems. To date, rationality and beliefs in dynamic games have not been subject to analysis from the revealed-preference perspective. Formal definitions of sequential rationality build upon expected-utility maximization. However, the revealed preference foundations for expected utility [14, 1] concern atemporal, or one-shot choices. Furthermore, extensions of expected-utility theory to dynamic choice problems are wholly silent about behavior conditional upon ex-ante zero probability events. Of course, the analysis of intended choices following unexpected moves is at the heart of dynamic game theory. Thus, the received decision theory is insufficient to provide foundations to the analysis of dynamic games.
The objective of this project is to provide such a foundation. This entails two contributions. The first is to define a novel choice criterion for dynamic decision problems and games, sequential preference, so as to satify two criteria. First, the proposed criterion implies sequential rationality. Second, it allows preferences over strategies to be elicited from ex-ante choices, using a version of the strategy method. In particular, sequential preferences provide a theoretical rationale for the use of this common experimental procedure, as well as a method to elicit conditional beliefs following zero-probability events.
Building on the finding that sequential preferences are indeed elicitable, the second contribution of this project is to provide a behavioral, or axiomatic, characterization of the proposed choice criterion. This is based on a suitable adaptation of the Anscombe-Aumann [1] axioms.
Finally, in the analysis of sequential preferences, there are natural connections to conditional probability systems [12,10] and lexicographic probability systems [2]. These are explored, and their implications for game-theoretic analysis are discussed. The project is carried out in two papers. The first, [18], introduces sequential preferences, and analyzes elicitation and the strategy method. The second, [17], provides behavioral foundations.