At least 18.5% of former National Football League players who died between 2008 and 2021 had chronic traumatic encephalopathy at death, according to a retrospective population-based cohort study published in The BMJ. That figure represents the floor rather than the ceiling: it is what the numbers show under the most conservative epidemiological assumptions the researchers could apply. In the later years of the study, from 2016 to 2021, when brain donation was most frequent, the minimum prevalence rose to 24.5%.
The design is what makes these numbers unusual. Estimating how common CTE is has been dogged by a basic problem - the brains available for autopsy tend to come from families who suspected something was wrong, which inflates apparent prevalence. To work around that, the researchers assembled a fully enumerated cohort of every identified former NFL player who died over the 14-year window, 1,712 men in total, then combined postmortem neuropathology from hundreds of donated brains with National Death Index records. Players whose brains were not examined were handled conservatively, which is why the results are framed as minimums.
The study also examined the clinicopathological link between CTE and dementia in this population, connecting the disease seen under the microscope with symptoms recorded during life. That matters for families and clinicians, who have had little firm ground on which to weigh the risk of a former player developing dementia, and for researchers designing studies of who is most vulnerable and why.
Even a conservative one-in-five to one-in-four figure for a defined professional population raises the question of what the true rate is once undiagnosed and unstudied cases are accounted for. The rise in the minimum estimate during the years with more brain donations suggests how much the picture depends on how many brains get examined, and how much remains unknown about players whose brains never reach a research lab.