  We present multi-epoch spectropolarimetry and spectra for a sample of 14 Type
IIn supernovae (SNe IIn). We find that after correcting for likely interstellar
polarization, SNe IIn commonly show intrinsic continuum polarization of 1--3%
at the time of peak optical luminosity, although a few show weaker or
negligible polarization. While some SNe IIn have even stronger polarization at
early times, their polarization tends to drop smoothly over several hundred
days after peak. We find a tendency for the intrinsic polarization to be
stronger at bluer wavelengths, especially at early times. While polarization
from an electron scattering region is expected to be grey, scattering of SN
light by dusty circumstellar material (CSM) may induce such a
wavelength-dependent polarization. For most SNe IIn, changes in polarization
degree and wavelength dependence are not accompanied by changes in the position
angle, requiring that asymmetric pre-SN mass loss had a persistent geometry.
While 2--3% polarization is typical, about 30% of SNe IIn have very low or
undetected polarization. Under the simplifying assumption that all SN IIn
progenitors have axisymmetric CSM (i.e. disk/torus/bipolar), then the
distribution of polarization values we observe is consistent with similarly
asymmetric CSM seen from a distribution of random viewing angles. This
asymmetry has very important implications for understanding the origin of
pre-SN mass loss in SNe IIn, suggesting that it was shaped by binary
interaction.
