  We present first results from RoboPol, a novel-design optical polarimeter
operating at the Skinakas Observatory in Crete. The data, taken during the May
- June 2013 commissioning of the instrument, constitute a single-epoch linear
polarization survey of a sample of gamma-ray - loud blazars, defined according
to unbiased and objective selection criteria, easily reproducible in
simulations, as well as a comparison sample of, otherwise similar, gamma-ray -
quiet blazars. As such, the results of this survey are appropriate for both
phenomenological population studies and for tests of theoretical population
models. We have measured polarization fractions as low as $0.015$ down to $R$
magnitude of 17 and as low as $0.035$ down to 18 magnitude. The hypothesis that
the polarization fractions of gamma-ray - loud and gamma-ray - quiet blazars
are drawn from the same distribution is rejected at the $10^{-3}$ level. We
therefore conclude that gamma-ray - loud and gamma-ray - quiet sources have
different optical polarization properties. This is the first time this
statistical difference is demonstrated in optical wavelengths. The polarization
fraction distributions of both samples are well-described by exponential
distributions with averages of $\langle p \rangle =6.4 ^{+0.9}_{-0.8}\times
10^{-2}$ for gamma-ray--loud blazars, and $\langle p \rangle =3.2
^{+2.0}_{-1.1}\times 10^{-2}$ for gamma-ray--quiet blazars. The most probable
value for the difference of the means is $3.4^{+1.5}_{-2.0}\times 10^{-2}$. The
distribution of polarization angles is statistically consistent with being
uniform.
