  We study the impact of lens galaxy environments on the image separation
distribution of lensed quasars. We account for both environmental convergence
and shear, using a joint distribution derived from galaxy formation models
calibrated by galaxy-galaxy lensing data and number counts of massive
elliptical galaxies. We find that the external field enhances lensing
probabilities, particularly at large image separations; the increase is ~30% at
\theta=3'' and ~200% at \theta=5'', when we adopt a power-law source luminosity
function \Phi(L) \propto L^-2.1. The enhancement is mainly driven by
convergence, which boosts both the image separation and magnification bias (for
a fixed lens galaxy mass). These effects have been neglected in previous
studies of lens statistics. Turning the problem around, we derive the posterior
convergence and shear distributions and point out that they are strong
functions of image separation; lens systems with larger image separations are
more likely to lie in dense environments.
