On manipulation in merging epistemic states.

In this work we study the problem of manipulation in the framework of merging complex epistemic states. We adopt the techniques concerning representation and impossibility in merging complex epistemic states proposed by Mata Diaz and Pino Perez in 2017. We introduce here the notion of belief lifting, aiming to capture the preferences of an agent over formulas. This allows us to define a general qualitative notion of manipulability. We prove that, given some rational properties, a merging operator is either manipulable, with respect to any well-behaved belief lifting, or it admits a powerful agent (a dictator or a nominator) who, in some way, imposes his will. We also study the behaviour of some concrete epistemic state merging operators, showing that most of them are manipulable. By means of this study, we prove that strategy-proofness cannot be characterised in terms of dictators or nominators in this qualitative framework of manipulability. (c) 2023 Elsevier Inc. All rights reserved.