Blue-solid histogram with error bars: metallicity distribution and its uncertainty of 29 0.3 <z< 1 GRB hosts (⟨ z ⟩ = 0.65). We applied a correction for the over-proportionality of dusty GRBs in the sample (Sects. 2.3.3 and 2.4) and for completeness from the line broadening. Black data show raw measurements. The red line is the expectation from the stellar-mass function at a similar redshift (Tomczak et al. 2014) if GRBs traced star formation without metallicity dependence. Shown curves are normalized to an area of 1.