Lineated magnetic anomalies indistinguishable from those generated at mid-oceanic ridges have been found behind many subduction zones. They show that back-arc spreading, like its mid-oceanic counterpart, is to a first approximation symmetric. Nevertheless, consistently asymmetric spreading has been found in some ocean basins, and in both actively spreading and extinct back-arc basins. We argue here that the sense of asymmetry found in active back-arc basins strongly supports a mechanism for the origin of asymmetric spreading which is dominated by the thermal effects of migration of the ridge crest over the sub-asthenospheric mantle. Conversely, it provides no support for the simple fluid mechanical models, nor for models in which back-arc extension is driven directly by the subduction process. If the thermal effect also dominated asymmetric spreading in extinct back-arc basins, the sense of asymmetry would provide information about `absolute' plate motions at the time of opening.