Whether the adult human brain can create new neurons has been a fundamental scientific question for years. Experimental evidence in non-human mammalian systems revealed that proliferation of new neurons in the adult hippocampus does occur under various conditions, while it is reduced or completely impeded in instances of neurologic disease. Making these inquiries in human subjects is not trivial, and for results gathered thus far it appears that the devil lies in the experimental details: how have different conclusions been reached in studies of neurogenesis in the human hippocampus? In a 2013 paper, scientists measured the presence of nuclear-bomb-test-generated 14C that was incorporated into the genomic DNA of human brains to retrospectively determine the birth dates of hippocampal cells; they reported that 700 new neurons are added per day in adult humans, followed by a modest decline during aging. Last year, it was reported that hippocampal neurogenesis drops sharply in children and is undetectable in adults. In this instance, a lack of immunostaining for a protein that is considered a reliable marker for immature neurons—“doublecortin (DCX)”—contributed to this conclusion. But recently, the authors of a Nature Medicine article refuted this conclusion of the 2018 study, arguing that prolonged chemical fixation of the studied brain tissues would have likely impeded the binding of DCX antibodies and impeded detection of the protein. They reported that longer than 48 hours of paraformaldehyde fixation reduced the identification of new neurons in human hippocampus samples, and greater than 6-months treatment would make their detection impossible; this corresponded with the fact that some samples from the 2018 study had been stored in paraformaldehyde for years. Authors of the 2019 report used a fixation time of only 24 hours to preserve donated brain tissue from 13 deceased adults—ranging in age from 43 to 87—and found tens of thousands of hippocampal DCX-positive cells at all ages. Whether or not detection of DCX protein will remain a sufficient standard to score neurons as “newly generated” remains unclear. But as the neuroscience field advances, vivid memories of these differences in experimental approach, and awareness of the need for real-time measurements of cellular proliferation in the human brain will be at the forefront of neuroscientists’ thinking.