The broadening and depolarization of pulsar average profiles may be due to the birefringence of the plasma above pulsar polar caps. The broadening at low radio frequency is caused by the separation of the individual beams of the two propagation modes, and the depolarization at high frequency results from the merger of their orthogonal polarizations. Observed spectra of pulse width and fractional linear polarization are consistent with this interpretation. Pulse profiles can depolarize rapidly with frequency when the linear polarizations of the beams are comparable. A comparison of the relative frequency of occurrence of the orthogonal modes, as observed in distributions of polarization position angle, with measured rates of depolarization suggests that pulsars depolarize via superposed modes of orthogonal radiation.