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 ≳ 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 SFRs for the highest redshift and highest SFR galaxies. In addition, this may resolve several outstanding problems in the chemical evolution of galactic haloes.