  We use a series of magnetohydrodynamic simulations including both radiative
and protostellar outflow feedback to study the environmental variation of the
initial mass function. The simulations represent a carefully-controlled
experiment whereby we keep all dimensionless parameters of the flow constant
except for those related to feedback. We show that radiation feedback
suppresses the formation of lower mass objects more effectively as the surface
density increases, but this only partially compensates for the decreasing Jeans
mass in denser environments. Similarly, we find that protostellar outflows are
more effective at suppressing the formation of massive stars in higher surface
density environments. The combined effect of these two trends is towards an IMF
with a lower characteristic mass and a narrower overall mass range in high
surface density environments. We discuss the implications of these findings for
the interpretation of observational evidence of IMF variation in early-type
galaxies.
