A spin field interacting with gravity in RW space-time cannot have a "standard-like" form

It is shown that if the Einstein field equation is coupled to the massive spinor field equation of spin s > 1/2, then the spin field cannot have the form of a standard solution nor to be superposition of standard solutions in Robertson Walker space-time. For standard solutions are meant the wide class of solutions of the spinor field equations previously determined by variable separation in RW space-time. The result is an extension to the fermion field case of a similar result previously obtained for massive bosons and for the Einstein-Dirac equation in RW space time.