  Previous surveys for HI 21-cm absorption in z > 0.1 radio galaxies and
quasars yield a 40% detection rate, which is attributed to unified schemes of
active galactic nuclei (AGN). In this paradigm absorption is only witnessed in
(close to) type-2 objects, where the central obscuration is viewed (nearly)
edge-on and thus absorbs the rest frame 1420 MHz emission along our sight-line.
However, we find this mix of detections and non-detections to only apply at low
redshift (z < 1): From a sensitive survey of eight z > 3 radio sources we find
no 21-cm absorption, indicating a low abundance of cold neutral gas in (the
sight-lines searched in) these objects. Analysing the spectral energy
distributions of these sources, we find that our high redshift selection
introduces a bias where our sample consists exclusively of quasars with
ultra-violet luminosities in excess of 10e23 W/Hz. This may suggest that we
have selected a class of particularly UV bright type-1 objects. Whatever the
cause, it must also be invoked to explain the non-detections in an equal number
of z < 0.7 sources, where we find, for the first time, the same exclusive
non-detections at > 10e23 W/Hz. These objects also turn out to be quasars and,
from these exclusive high UV luminosity--21-cm non-detections, it is apparent
that orientation effects alone cannot account for the mix of 21-cm detections
and non-detections at any redshift.
