  The less depolarized lobe of a radio source is generally the lobe containing
the jet (Laing-Garrington correlation) but the less depolarized lobe is also
generally that with the flatter radio spectrum (Liu-Pooley correlation). Both
effects are strong; taken together they would imply a correlation between jet
side and lobe spectral index, i.e. between an orientation-dependent feature and
one which is intrinsic. We test this prediction using detailed spectral imaging
of a sample of quasars with well-defined jets and investigate whether the
result can be reconciled with the standard interpretation of one-sided jets in
terms of relativistic aberration. Our central finding is that the spectrum of
high surface brightness regions is indeed flatter on the jet side, but that the
spectrum of low surface brightness regions is flatter on the side with the
longer lobe. We discuss possible causes for these correlations and favour
explanations in terms of relativistic bulk motion in the high surface
brightness regions and differential synchrotron ageing in the extended lobe
material.
