  Only galaxies bright enough and large enough to be unambiguously identified
and measured are included in galaxy surveys used to estimate cosmic shear. We
demonstrate that because gravitational lensing can scatter galaxies across the
brightness and size thresholds, cosmic shear experiments suffer from lensing
bias. We calculate the effect on the shear power spectrum and show that -
unless corrected for - it will lead analysts to cosmological parameters
estimates that are biased at the 2-3\sigma level in DETF Stage III experiments,
such as the Dark Energy Survey.
