A Coalitional Game-Theoretic Model of Stable Government Forms with Umpires

In this paper a government form is modeled as an effectivity function scheme (EFS) i.e. a parameterized family of effectivity functions having admissible weight-profiles of assemblies as the relevant parameters. Working in a 2-jurisdiction outcome space we show that the existence of umpires is consistent with strong core-stability of both neo-parliamentary and semi-presidential government forms provided that the majority formation rule is collegial, namely there is one electorally fixed minimal majority coalition.