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Following the discovery of the first significant samples of galaxies at  z  &gt; 6.5 with Wide Field Camera 3/Infra‐Red (WFC3/IR) on board  Hubble Space Telescope  ( HST ), it has been claimed that the faintest high‐redshift galaxies display extremely blue ultraviolet (UV) continuum slopes, with a UV power‐law index β≃−3 (where  f  λ ∝λ β ). Such slopes are bluer than previously reported for any other galaxy population, and are most readily explained theoretically by extinction‐free, young and very low metallicity stellar populations with a high ionizing photon escape fraction. Here we undertake a critical study of the evidence for such extreme values of β, combining three new WFC3/IR‐selected samples of galaxies spanning nearly two decades in UV luminosity over the redshift range  z ≃ 4.5–8. We explore the impact of inclusion/exclusion of less robust high‐redshift candidates and use the varying depths of the samples to explore the effects of noise and selection bias at a given UV luminosity. Simple data‐consistency arguments suggest that artificially blue average values of β can result when the analysis is extended into the deepest ≃0.5 mag bin of these WFC3/IR‐selected galaxy samples, regardless of the actual luminosity or redshift range probed. By confining attention to robust high‐redshift galaxy candidates, with at least one 8σ detection in the WFC3/IR imaging, we find that the average value of β is consistent with 〈β〉=−2.05 ± 0.10 over the redshift range  z = 5–7 and the UV absolute magnitude range  −22 &lt;  M  UV,AB  &lt; − 18 , and that 〈β〉 shows no significant trend with either redshift or   M  UV  . We create and analyse a set of simple end‐to‐end simulations based on the WFC3/IR+ACS  Hubble Ultra Deep Field  (HUDF) and Early Release Science data sets which demonstrate that a bias towards artificially low/blue average values of β is indeed ‘expected’ when the UV slope analysis is extended towards the source detection threshold, and conclude that there is as yet no clear evidence for UV slopes significantly bluer than β≃−2, the typical value displayed by the bluest star‐forming galaxies at more modest redshifts. A robust measurement of 〈β〉 for the faintest galaxies at  z ≃ 7 (and indeed  z ≃ 8) remains a key observational goal, as it provides a fundamental test for high escape fractions from a potentially abundant source of re‐ionizing photons. This goal is achievable with  HST , but requires still deeper WFC3/IR imaging in the HUDF.

A critical analysis of the ultraviolet continuum slopes (β) of high‐redshift galaxies: no evidence (yet) for extreme stellar populations at z > 6