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Rosas et al. (Reports, 22 September 2017, p. 1282) calculate El Sidrón J1 to have reached only 87.5% of its adult brain size. This finding is based on an overestimation of Neandertal brain size. Pairwise comparisons with a larger sample of Neandertal fossils reveal that it is unlikely that the brain of El Sidrón would have grown appreciably larger.