  A supermassive black hole has been found at the centre of nearly every galaxy
observed with sufficient sensitivity. The masses of these black holes are
observed to increase with either the total mass or the mean (random) velocity
of the stars in their host galaxies. The origin of these correlations remains
elusive. Observational systematics and biases severely limit our knowledge of
the local demography of supermassive black holes thus preventing accurate model
comparisons and progress in this field. Here we show that the large-scale
spatial distribution of local active galactic nuclei (AGN), believed to be
accreting supermassive black holes, can constrain the shape and normalization
of the black hole-stellar mass relation thus bypassing resolution-related
observational biases. In turn, our results can set more stringent constraints
on the so-called "radiative efficiency", a fundamental parameter describing the
inner physics of supermassive black holes that is closely linked to their spin,
geometry, and ability to release energy. The mean value of the radiative
efficiency can be estimated by comparing the average total luminous output of
AGN with the relic mass density locked up in quiescent supermassive black holes
at galaxy centres today. For currently accepted values of the AGN obscured
fractions and bolometric corrections, our newest estimates of the local
supermassive black hole mass density favour mean radiative efficiencies of
~10-20\%, suggesting that the vast majority of supermassive black holes are
spinning moderately to rapidly. With large-scale AGN surveys coming online, our
novel methodology will enable even tighter constraints on the fundamental
parameters that regulate the growth of supermassive black holes.
