additional support for models dealing with the joint evolution of QSOs and normal galaxies \[e.g. see @hk00 [@granato; @xfranc] and references therein\]. In particular, the fact that the correlation length of AGN at $z\sim 0$ is consistent with that of ellipticals or S0 in the local universe, reinforces the hypothesis that the QSO host galaxy should be old.

Comparison with QSO clustering at high-redshift
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@lac98 investigated the evolution of QSO clustering using a sample of 388 QSOs with $0.3\le z\le 2.2$ over a connected area of 25 deg$^2$ down to $B\le 20.5$ magnitude. Evidence was found for an increase of the clustering with increasing redshift ($\bar{\xi}(20)=0.22^{+0.28}_{-0.18}$ at $0.3\le z\le 1.4$ and $\bar{\xi}(20)=0.87^{+0.38}_{-0.21}$ at $1.4\le z\le 2.2$). This result does not support the idea of a single population model for QSOs. The general properties of the QSO population studied by @lac98 would arise naturally if QSOs are short-lived events ($\tau \sim 10^{6}-10^{7}$ yr) related to a characteristic halo mass of $\sim 5\cdot 10^{12} M_{\odot}$.

@2qzclust have used more than 10,000 QSOs taken from the preliminary data release catalog of 2QZ to measure the QSO clustering as a function of redshift. Their sample spans two connected areas for a total of 