  The recently developed pragmatical model of asymmetric double-well potentials
with a finite lifetime is applied to nonlinear dielectric data in polar
undercooled liquids. The viscous effects from the finite lifetime provide a
crossover from the cooperative jumps of many molecules at short times to the
motion of statistically independent molecules at long times. The model allows
to determine the size of cooperatively rearranging regions from nonlinear
{\omega}-data and throws a new light on a known inconsistency between nonlinear
{\omega} and 3{\omega}-signals for glycerol and propylene carbonate.
