  A reliable model of galaxy bias is necessary for interpreting data from
future dense galaxy surveys. Conventional bias models are inaccurate, in that
they can yield unphysical results ($\delta_g < -1$) for voids that might
contain half of the available cosmological information. For this reason, we
present a physically-motivated bias model based on an analogy with the Ising
model. With only two free parameters, the model produces sensible results for
both high- and low-density regions. We also test the model using a catalog of
Millennium Simulation galaxies in cubical survey pixels with side lengths from
$2h^{-1}$--$31h^{-1}$Mpc, at redshifts from 0 to 2. We find the Ising model
markedly superior to linear and quadratic bias models on scales smaller than
$10h^{-1}$Mpc, while those conventional models fare better on scales larger
than $30h^{-1}$Mpc. While the largest scale where the Ising model is applicable
might vary for a specific galaxy catalog, it should be superior on any scale
with a non-negligible fraction of cells devoid of galaxies.
