Ghost seats in parliaments

•We study an electoral reform.•We allow for a variable number of seats in Parliaments.•The total number of seats depends on the number of dissatisfaction votes.•We quantify the effect of the electoral reform with power indices.•We illustrate the effect by the case of the Basque Parliament.

In several European Countries citizens' movements have formed parties whose programme is to reform electoral legislation so that blank votes are transformed into vacant seats in parliament. The first objective of this paper is to offer some tools in order to quantify the potential effect of a reform that would allow citizens to cast a dissatisfaction vote. We show that such a reform would reduce the ease to pass a proposal and modify the distribution of power among parties. The second objective of this paper is to show that this reform could have an impact in parliaments. This is illustrated by the case of the Basque Parliament from 1980 to 2016. At the theoretical level this paper provides some relations between measures of success.