  We derive an analytic expression for the mean magnification due to strong
gravitational lensing, using a simple lens model, a singular isothermal sphere
embedded in an external shear field. We compute separate expressions for
2-image and 4-image lensing. For 4-image lensing, the mean magnification takes
a particularly simple form: <mu>_4 = 3.6/(gamma*(1-gamma^2)), where gamma is
the external shear. We compare our analytic results to a numerical evaluation
of the full magnification distribution. The results can be used to understand
the magnification bias that favors the discovery of four-image systems over
two-image systems in flux-limited lens surveys.
