Dutch eat at 5:30 pm: Shared Strategies in Agent Systems A. Ghorbani 1 , H. Aldewereld 1 , V. Dignum 1 , and P. Noriega 2 1 Delft University of Technology, Faculty of Technology, Policy and Management, Delft, The Netherlands (a.ghorbani,H.M.Aldewereld,M.V.Dignum)@tudelft.nl 2 Artificial Intelligence Research Institute of the Spanish National Scientific Research Council,Barcelona, Spain, pablo@iiia.csic.es Abstract. A shared strategy is a social concept that refers to a type of behavioural pattern that is followed by a significant number of individ- uals although it is, prima facie, not associated with an obligation or a prohibition. E. Ostrom has argued in favour of the pertinence of social strategies for institutional design and evolution and proposed a charac- terization suggestive of formal treatment. However, shared strategies as such have not been explicitly used in the context of regulated MAS in spite of their relevance and their affinity to more standard normative notions, of which a rich tradition exists in MAS research. In this paper, we discuss the notion of shared strategy, characterize its distinguishing features, propose its formalization using a temporal epistemic logic, and explore its potential use in regulated multi-agent systems. 1 Introduction In the Netherlands, almost all people have dinner around 5:30pm. As a foreigner in that country, it is almost impossible to plan a (working) meeting around this time, which would be a ‘normal’ time in many other countries. On the other hand, having dinner that early is not an obligation. No one will be offended or would even care if you choose to eat later. In other words, there is no particular goal that everyone must reach following this strategy and therefore, individual disobedience would not have any particular consequence. One other important attribute of such statement is that it is more significant at the collective level rather than individual. In many cases people are not even aware of the common behaviour they are showing. Therefore, it is not a decision making action but rather more a routine-based reactive process. Nevertheless, knowledge of this typically Dutch behaviour, can help actors to plan their own, or joint activities (e.g., you can go to the supermarket at that time as it is likely to be very quiet, or you can invite your Dutch friends for dinner at that time). This is an example of a shared strategy, i.e. an institutional arrangement where different actors have the intention of performing the same task at a certain time or setting [15]. Even though the concept of shared strategy is socially and computationally very instrumental, it has not yet been implemented nor formalized in the MAS