  Beam asymmetries result in statistically-anisotropic cosmic microwave
background (CMB) maps. Typically, they are studied for their effects on the CMB
power spectrum, however they more closely mimic anisotropic effects such as
gravitational lensing and primordial power asymmetry. We discuss tools for
studying the effects of beam asymmetry on general quadratic estimators of
anisotropy, analytically for full-sky observations as well as in the analysis
of realistic data. We demonstrate this methodology in application to a
recently-detected 9 sigma quadrupolar modulation effect in the WMAP data,
showing that beams provide a complete and sufficient explanation for the
anomaly.
