  In purely c-axis oriented PbZr$_{0.2}$Ti$_{0.8}$O$_3$ ferroelectric thin
films, a lateral piezoresponse force microscopy signal is observed at the
position of 180{\deg}domain walls, where the out-of-plane oriented polarization
is reversed. Using electric force microscopy measurements we exclude
electrostatic effects as the origin of this signal. Moreover, our mechanical
simulations of the tip/cantilever system show that the small tilt of the
surface at the domain wall below the tip does not satisfactorily explain the
observed signal either. We thus attribute this lateral piezoresponse at domain
walls to their sideways motion (shear) under the applied electric field. From
simple elastic considerations and the conservation of volume of the unit cell,
we would expect a similar lateral signal more generally in other ferroelectric
materials, and for all types of domain walls in which the out-of-plane
component of the polarization is reversed through the domain wall. We show that
in BiFeO$_3$ thin films, with 180, 109 and 71{\deg}domain walls, this is indeed
the case.
