  Spins play a crucial role in the appearance, evolution and occupation
fraction of massive black holes. To date observational estimates of massive
black hole spins are scarce, and the assumptions commonly made in such
estimates have been recently questioned. Similarly, theoretical models for
massive black hole spin evolution, while reproducing the few observational
constraints, are based on possibly oversimplified assumptions. New independent
constraints on massive black hole spins are therefore of primary importance. We
present a rigorous statistical analysis of the relative orientation of radio
jets and mega-maser discs in ten low-redshift galaxies. We find a strong
preference for (partial) alignment between jets and mega-masers, that can be
attributed to two different causes: "coherent" and "selective" accretion. In
the first case the partial alignment is due to an anisotropy in the gas
reservoir fueling the growth of MBHs. In the second case the spin-dependent
anisotropic feedback allows for long-lived accretion only if the orbits of the
gas inflows are almost aligned to the MBH equatorial plane. A discussion of the
implications of the two accretion scenarios on the evolution of MBHs is
presented, together with an outlook on future observational tests aiming at
discriminating between the two scenarios and at checking if any of the two
apply to different redshifts and black hole mass regimes.
