  The evolution of QSO clustering is investigated with a new sample of 388 QSOs
with 0.3<z<2.2, B<20.5 and M_B<-23. Evidence is found for an increase of the
clustering amplitude with increasing redshift. These measurements allow to
further distinguish among the various physical scenarios proposed to interpret
the QSO phenomenon. A single population model is inconsistent with the
observations. The general properties of the QSO population would arise
naturally if quasars are short-lived events connected to a characteristic halo
mass ~5x10^{12} M_sun.
