In this paper, we try to explain the intergovernmental negotiation of the Lisbon treaty from a rational choice perspective with the aid of power index analysis. There are two aspects of the reform of qualified majority voting (QMV) in the Council that we find puzzling. The first one is that, according to Shapley-Shubik index based on the notion of power as the distribution of a fixed prize, small and medium-sized member states have lost power as compared to the Nice treaty, which conflicts with

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... the unanimity requirement for treaty reform. The second one is that, according to the Banzhaf measure based on the notion of power as influence, the Lisbon treaty leaves all member states worse off in absolute terms as compared to the Convention's draft. We propose the measure developed by Steunenberg et al. (1999) as a possible solution to these paradoxes, and draw some conclusions about the nature of EU policy making and power index analysis.