  Dielectric permittivity and Second Harmonic Generation (SHG) studies in the
field-cooled mode show a linear dependence of dielectric stiffness (inverse
dielectric permittivity) on dc bias in PMN-PT crystals and SHG intensity in
KTaO$_{3}$:Li at small Li concentrations. We explain this unusual result in the
framework of a theory of transverse, hydrodynamic-type, instability of local
polarization.
