Massive Gravity Simplified: A Quadratic Action
  We present a simplified formulation of massive gravity where the Higgs fields have quadratic kinetic term. This new formulation allows us to prove in a very explicit way that all massive gravity theories considered so far inevitably have Boulware-Deser ghost in non-trivial fluctuations of background metric. 
Ali H. Chamseddine and Viatcheslav Mukhanov
hep-th gr-qc