  We argue that an increased temperature in star-forming clouds alters the
stellar initial mass function to be more bottom-light than in the Milky Way. At
redshifts $z \gtrsim 6$, heating from the cosmic microwave background radiation
produces this effect in all galaxies, and it is also present at lower redshifts
in galaxies with very high star formation rates (SFRs). A failure to account
for it means that at present, photometric template fitting likely overestimates
stellar masses and star formation rates for the highest-redshift and
highest-SFR galaxies. In addition this may resolve several outstanding problems
in the chemical evolution of galactic halos.
