Recent measurements of high-redshift QSO clustering from the Sloan Digital Sky Survey indicate that QSOs at z \sim 4 have a bias \langle b \rangle \simeq 14 .
We find that this extremely high clustering amplitude , combined with the corresponding space density , constrains the dispersion in the L - M _ { h } relation to be less than 50 % at 99 % confidence for the most conservative case of a 100 % duty cycle .
This upper limit to the intrinsic dispersion provides as strong a constraint as current upper limits to the intrinsic dispersion in the local M _ { BH } - \sigma relation and the ratio of bolometric to Eddington luminosity of luminous QSOs .