  Gamma ray burst (GRB) optical and X-ray afterglow luminosity is expected to
correlate with the GRB isotropic equivalent kinetic energy of the outflow in
the standard synchrotron model for GRB afterglows. Previous studies, using
prompt GRB isotropic equivalent energy ($E_{\rm iso}$) as a proxy for isotropic
equivalent kinetic energy, have generally confirmed a correlation between X-ray
and optical afterglow luminosities. Assuming that GRB afterglow luminosity does
not evolve strongly with redshift, we identify a strong Malmquist bias in GRB
optical and X-ray afterglow luminosity data. We show that selection effects
dominate the observed E$_{\rm iso}$ -- L$_{\rm opt,X}$ correlations, and have
likely been underestimated in other studies. The bias is strongest for a subset
of optically faint bursts $m>24$ at 24 hr with $z>2$. After removing this
optical selection bias, the E$_{\rm iso}$ -- L$_{\rm opt,X}$ correlation for
long GRBs is not statistically significant, but combining both long and short
GRB luminosity data the correlation is significant. Using the median of the
$E_{\rm iso}$ and $L_{\rm opt,X}$ distributions, we apply the synchrotron model
assuming the same power law index for short and long GRBs, but different
microphysical parameter distributions. Comparing the ratio of optical and X-ray
luminosities, we find tentative evidence that the fraction of post-shock energy
in magnetic fields, $\epsilon_B$, could be systematically higher in SGRBs
compared to LGRBs.
