Evolution in the comoving number densities of Type 1 quasars at four different values of M _BH. The bars contain 68% of the posterior probability. The green bars denote bins with Hβ-based virial mass estimates, the blue bars denote bins with Mg ii-based mass estimates, and the red bars denote bins with C iv-based mass estimates. All of the bins show a peak in the number density around z ~ 2, with the higher mass bins falling off more steeply toward lower redshift and less steeply toward higher redshift, relative to the peak. The trend is commonly referred to as "downsizing."