  We show that the correct perturbation theory for mixed fermion states leads
to nonvanishing contributions of the dimension-four vertex operators for
flavour-changing transitions. Their contributions to the amplitude are of the
same order of magnitude as the dimension- five vertex operators. Considerations
are valid irrespective of the electroweak model. A complete and correct
evaluation of the neutrino lifetime for particular electroweak models could be
of greate importance in the theoretical cosmology and astrophysics.
