We use a sample of 4209 Lyman-break galaxies (LBGs) at z 3, 4, and 5 in the UKIRT
Infrared Deep Sky Survey Ultra Deep Survey field to investigate the relationship between the
observed slope of the stellar continuum emission in the ultraviolet, β, and the thermal dust
emission, as quantified via the so-called ‘infrared excess’ (IRX ≡ LIR/LUV). Through a stacking
analysis, we directly measure the 850-μm flux density of LBGs in our deep (0.9 mJy) James
Clerk Maxwell Telescope SCUBA-2 850-μm map as well as deep public Herschel/SPIRE
250-, 350-, and 500-μm imaging. We establish functional forms for the IRX–β relation to
z ∼ 5, confirming that there is no significant redshift evolution of the relation, and that the
resulting average IRX–β curve is consistent with a Calzetti-like attenuation law. Comparing
our results with recent works in the literature, we confirm that discrepancies in the slope of
the IRX–β relation are driven by biases in the methodology used to determine the ultraviolet
slopes. Consistent results are found when IRX–β is evaluated by stacking in bins of stellar
mass, and we argue that the near-linear IRX–M relationship is a better proxy for correcting
observed ultraviolet luminosities to total star formation rates, provided an accurate handle on
M and also gives clues as to the physical driver of the role of dust-obscured star formation in
high-redshift galaxie