Monotonicity: Difference between revisions

A party-list strategy exploiting something similar (down-ranking CDU '''and''' additionally up-ranking another party, e.g. FDP) happened in the German federal election of 2005, in which conservative voters in Dresden deliberately voted against the CDU, their party of choice, in order to maximize that party's number of seats in the federal parliament. This was possible due to Germany's voting system (mixed member proportional with overhang seats computed independently for each federal state) and the fact that the vote in Dresden took place a week after the rest of the country due to the death of a candidate, enabling voters in Dresden to vote tactically in full knowledge of the results already achieved elsewhere. As a result of this, the German Constitutional Court ruled on July 3 2008 that the German voting system must be reformed to eliminate its non-monotonicity.<ref> See e.g. [http://fruitsandvotes.com/blog/?p=117]</ref>

 

 

A real-life monotonicity violation was detected in the [[Burlington, Vermont mayoral election, 2009|2009 Burlington, Vermont mayor election]] under instant-runoff voting (IRV), where the necessary information is available. In this election, the winner Bob Kiss could have been defeated by raising him on some of the ballots. For example, if all voters who ranked Kurt Wright over Bob Kiss over Andy Montroll, would have ranked Kiss over Wright over Montroll, and additionally some people who ranked Wright but not Kiss or Montroll, would have ranked Kiss over Wright, then these votes in favor of Kiss would have defeated him.<ref>[http://www.rangevoting.org/Burlington.html Burlington Vermont 2009 IRV mayor election]</ref> The winner in this scenario would have been Andy Montroll, who was also the [[Condorcet winner]] according to the original ballots, i.e. for any other running candidate, a majority ranked Montroll above the competitor.