  Recent work on the engines of active galactic nuclei jets suggests their
power depends strongly and perhaps counter-intuitively on black hole spin. We
explore the consequences of this on the radio-loud population of active
galactic nuclei and find that the time evolution of the most powerful radio
galaxies and radio-loud quasars fits into a picture in which black hole spin
varies from retrograde to prograde with respect to the accreting material.
Unlike the current view, according to which jet powers decrease in tandem with
a global downsizing effect, we argue for a drop in jet power resulting directly
from the paucity of retrograde accretion systems at lower redshift $z$ caused
by a continuous history of accretion dating back to higher $z$. In addition,
the model provides simple interpretations for the basic spectral features
differentiating radio-loud and radio-quiet objects, such as the presence or
absence of disk reflection, broadened iron lines and signatures of disk winds.
We also briefly describe our models' interpretation of microquasar state
transitions. We highlight our result that the most radio-loud and most
radio-quiet objects both harbor highly spinning black holes but in retrograde
and prograde configurations, respectively.
