  Electrostrictive materials exhibit a strain that is proportional to the
square of the induced polarization. In linear dielectrics where the
permittivity is constant, this electromechanical strain is also proportional to
the square of the electric field. However, under increasing amplitudes of the
driving field, the electromechanical strain sometimes saturates; the
electrostrictive coefficients therefore appear to depend on the amplitude of
the electric field used to measure them. Here, we present a methodology showing
that this apparent field dependence is a consequence of neglecting higher-order
electromechanical phenomena. When these are taken into account, not only do the
electrostrictive coefficients remain constant but the signs of the high-order
coefficients enable the prediction of the saturation behavior from a single
measurement. We illustrate this approach on both classical and non-classical
(so-called ``giant'') electrostrictors.
