The description of plate motions in the so-called hotspot reference frame introduces a global rotation of the lithosphere with respect to the mantle. This rotation, called toroidal field of degree 1, is roughly westward. It reaches an amplitude of about 2 cm/yr and has been consistently found in the different generation of plate tectonic models. Various authors have tried to relate this observation to the deceleration of the Earth's rotation, to polar wander, or to tidal drag. However, these different physical mechanisms cannot explain the requested amplitude. In this paper, we compare the values of this rotation vector using different relative plate motion models expressed in the hotspot reference frame. In a model Earth with lateral viscosity variations, a differential rotation is predicted. The observed net lithospheric rotation is consistent with the dynamics of a model Earth where the asthenospheric viscosity below the oceans is at least one order of magnitude lower than underneath the continents. This relative westward drift of the lithosphere may account for the significant structural differences between east or west dipping subduction zones. ¿American Geophysical Union 1991