  We have conducted the largest investigation to date into the origin of phase
resolved, apparent RM variations in the polarized signals of radio pulsars.
From a sample of 98 pulsars based on observations at 1.4 GHz with the Parkes
radio telescope, we carefully quantified systematic and statistical errors on
the measured RMs. A total of 42 pulsars showed significant phase resolved RM
variations. We show that both magnetospheric and scattering effects can cause
these apparent variations. There is a clear correlation between complex
profiles and the degree of RM variability, in addition to deviations from the
Faraday law. Therefore, we conclude that scattering cannot be the only cause of
RM variations, and show clear examples where magnetospheric effects dominate.
It is likely that, given sufficient signal-to-noise, such effects will be
present in all radio pulsars. These signatures provide a tool to probe the
propagation of the radio emission through the magnetosphere.
