  Recent observations of the black hole (BH) - bulge scaling relations usually
report positive redshift evolution, with higher redshift galaxies harboring
more massive BHs than expected from the local relations. All of these studies
focus on broad line quasars with BH mass estimated from virial estimators based
on single-epoch spectra. Since the sample selection is largely based on quasar
luminosity, the cosmic scatter in the BH-bulge relation introduces a
statistical bias leading to on average more massive BHs given galaxy properties
at high redshift (Lauer et al. 2007). We here emphasize a previously
under-appreciated statistical bias resulting from the uncertainty of
single-epoch virial BH mass estimators and the shape of the underlying (true)
BH mass function, which leads to on average overestimation of the true BH
masses at the high-mass end (Shen et al. 2008). We demonstrate that the latter
virial mass bias can contribute a substantial amount to the observed excess in
BH mass at fixed bulge properties, comparable to the Lauer et al. bias. The
virial mass bias is independent of the Lauer et al. bias, hence if both biases
are at work, they can largely (or even fully) account for the observed BH mass
excess at high redshift.
