  We investigate the accretion-driven growth of supermassive black holes in the
low-redshift Universe using 23,000 ``Type 2'' AGN and the complete sample of
123,000 galaxies in the SDSS from which they were drawn. We use the stellar
velocity dispersions of the early type galaxies and AGN hosts to estimate their
black hole masses and we use the AGN [OIII]5007 emission line luminosities to
estimate black hole accretion rates. We find that most present-day accretion
occurs onto black holes with masses < 10E8 Msun that reside in moderately
massive galaxies (M ~ 10E10 to 10E11.5 Msun) with high stellar densities
(~10E8.5 to 10E9.5 Msun/kpc^2) and young stellar populations. The
volume-averaged accretion rates of low mass black holes (<10E7.5 Msun) imply
that this population is growing on a timescale that is comparable to the age of
the Universe. Around half this growth takes place in AGN that are radiating
within a factor of five of the Eddington luminosity. Such systems are rare,
making up only 0.2% of the low mass black hole population at the present day.
The rest of the growth occurs in lower luminosity AGN. The growth timescale is
more than two orders of magnitude longer for the population of the most massive
black holes in our sample. The volume averaged ratio of star formation to black
hole accretion in bulge-dominated galaxies is ~1000, in remarkable agreement
with the observed ratio of stellar mass to black hole mass in nearby galaxy
bulges. We conclude: a) that bulge formation and black hole formation are
tightly coupled, even in present-day galaxies; and b) that the evolution of the
AGN luminosity function documented in recent optical and x-ray surveys is
driven by a decrease in the characteristic mass scale of accreting black holes.
