  Star-forming galaxies at $z > 1$ exhibit significantly different properties
to local galaxies of equivalent stellar mass. Not only are high-redshift
star-forming galaxies characterized by higher star formation rates and gas
fractions than their local counterparts, they also appear to host star-forming
regions with significantly different physical conditions, including greater
electron densities. To understand what physical mechanisms are responsible for
the observed evolution of star-forming conditions we have assembled the largest
sample of star-forming galaxies at $z\sim 1.5$ with emission-line measurements
of the $\mathrm{[OII]} \lambda \lambda 3726,3729$ doublet. By comparing our
$z\sim 1.5$ sample to local galaxy samples with equivalent distributions of
stellar mass, star formation rate and specific star formation rate we
investigate the proposed evolution in electron density and its dependence on
global properties. We measure an average electron density of $114_{-27}^{+28}
\, \mathrm {cm}^{-3} $ for our $z\sim 1.5$ sample, a factor of five greater
than the typical electron density of local star-forming galaxies. However, we
find no offset between the typical electron densities of local and
high-redshift galaxies with equivalent star-formation rates. Our work indicates
that the average electron density of a sample is highly sensitive to the star
formation rates, implying that the previously observed evolution is mainly the
result of selection effects.
