  Two spectacular results emerging from recent studies of nearby dead quasars
and distant active quasars are (i) the existence of tight relations between the
masses of black holes (BHs) and the properties of their host galaxies (spheroid
luminosity or velocity dispersion, galaxy mass), and (ii) a consistency between
the local mass density in BHs and that expected by summing up the light
received from distant active quasars. These results are partly shaped by
successive galactic mergers and BH coalescences, since mergers redistribute the
population of BHs in galaxies and BH binary coalescences reduce the mass
density in BHs through losses to gravitational waves. Here, we isolate and
quantify these effects by following the cosmological merger history of a
population of massive BHs representing the quasar population between z = 3 and
z =0. Our results suggest that the relation between BH mass and host galaxy
properties, and inferences on the global efficiency of BH accretion during
active quasar phases, could be influenced by the cumulative effect of repeated
mergers.
