  We present circular polarization results from a 5 GHz survey of the
parsec-scale polarization properties of 40 AGN made with the VLBA. We find 11
circular polarization detections at the 3 sigma level or higher. This nearly
quadruples the number of sources detected in circular polarization at VLBI
resolution. We find no correlation between fractional linear and circular
polarization across our sample. A likely explanation is external Faraday
depolarization in the cores of AGN which reduces linear polarization but leaves
circular polarization unchanged. In comparing ours and other recent results to
observations made 20 years ago, we find that, in five of six cases, sources
have the same sign of circular polarization today as they did 20 years ago.
This suggests the presence of a long term property of the jets, such as the
polarity of a net magnetic flux, which is stable on time-scales much longer
than those of individual outbursts.
