Helsinki Algorithms Seminar: Jan Hazla, MIT "Reasoning in Bayesian Opinion Exchange Networks is Computationally Hard"

Weekly meeting of researchers in the Helsinki area interested in the art of algorithms and algorithm design.
Department of Computer Science, image: Matti Ahlgren

Reasoning in Bayesian Opinion Exchange Networks is Computationally Hard

Jan Hazla
MIT Institute for Data, Systems and Society (IDSS)


Bayesian models of opinion exchange are extensively studied in economics, dating back to the work of Aumann on the agreement theorem. An important class of such models features agents arranged on a network (representing, e.g., social interactions), with the network structure determining which agents communicate with each other. It is often argued that the Bayesian computations needed by agents in such models are difficult, but prior to our work there were no rigorous arguments for such hardness.

We consider a well-studied model where fully rational agents receive private signals indicative of an unknown state of the world. Then, they repeatedly announce the state of the world they consider most likely to their neighbours, at the same time updating their beliefs based on their neighbours’ announcements.

I will discuss our complexity-theoretic results establishing hardness of agents’ computations in this model. Specifically, we show that these computations are NP-hard and extend this result to PSPACE-hardness. We show hardness not only for exact computations, but also that it is computationally difficult even to approximate the rational opinion in any meaningful way.

Joint work with Ali Jadbababie, Elchanan Mossel and Amin Rahimian.


Helsinki Algorithms Seminar is a weekly meeting of researchers in the Helsinki area interested in the art of algorithms and algorithm design, broadly interpreted to cover both theoretical ideas and algorithm engineering on concrete computing platforms. In most cases we have a presentation prepared for each meeting to communicate an idea, a recent result, work-in-progress, or demo, but this should not be at the expense of discussion and simply having fun with algorithms.

Our affiliations are with Aalto University and the University of Helsinki, and accordingly our activities alternate between the Otaniemi Campus of Aalto University and the Kumpula Campus of University of Helsinki, catalyzed by the Helsinki Institute for Information Technology HIIT, under the Algorithmic Data Analysis (ADA) programme.

For the season programme, please see the seminar webpage.

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