  In powerful radio-quiet active galactic nuclei (AGN), black holes heavier
than one billion solar masses form at a redshift ~1.5-2. Supermassive black
holes in jetted radio-loud AGN seems to form earlier, at a redshift close to 4.
The ratio of active radio-loud to radio-quiet AGN hosting heavy black holes is
therefore a rather a strong function of redshift. We report on some recent
evidence supporting this conclusion, gathered from the Burst Alert Telescope
(BAT, onboard Swift) and by the Large Area Telescope (LAT, onboard Fermi). We
suggest that the more frequent occurrence of relativistic jets in the most
massive black holes at high redshifts, compared to later times, could be due to
the average black hole spin being greater in the distant past, or else to the
jet helping a fast accretion rate (or some combination of the two scenarios).
We emphasize that the large total accretion efficiency of rapidly spinning
black holes inhibits a fast growth, unless a large fraction of the available
gravitational energy of the accreted mass is not converted into radiation, but
used to form and maintain a powerful jet.
