  We investigate the evolution of both the radio-loud fraction (RLF) and (using
stacking analysis) the mean radio-loudness of quasars. We consider how these
values evolve as a function of redshift and luminosity, black hole (BH) mass
and accretion rate, and parameters related to the dominance of a wind in the
broad emission line region. We match the FIRST source catalog to samples of
luminous quasars (both spectroscopic and photometric), primarily from the Sloan
Digital Sky Survey. After accounting for catastrophic errors in BH mass
estimates at high-redshift, we find that both the RLF and the mean radio
luminosity increase for increasing BH mass and decreasing accretion rate.
Similarly both the RLF and mean radio loudness increase for quasars which are
argued to have weaker radiation line driven wind components of the broad
emission line region. In agreement with past work, we find that the RLF
increases with increasing luminosity and decreasing redshift while the mean
radio-loudness evolves in the exact opposite manner. This difference in
behavior between the mean radio-loudness and the RLF in L-z may indicate
selection effects that bias our understanding of the evolution of the RLF;
deeper surveys in the optical and radio are needed to resolve this discrepancy.
Finally, we argue that radio-loud (RL) and radio-quiet (RQ) quasars may be
parallel sequences but where only RQ quasars at one extreme of the distribution
are likely to become RL, possibly through slight differences in spin and/or
merger history.
