BOSTON – Families worried about a former football player’s memory or behavior are more likely to donate his brain for research. That has long complicated a central question about the NFL’s head-injury legacy: how much disease exists beyond the players whose brains scientists actually examine?
A Boston University-led study published Wednesday estimates that 74.5% of former NFL players who died between 2008 and 2023 had chronic traumatic encephalopathy, or CTE, pathology at death. The researchers reached that figure after adjusting for differences between brain donors and other deceased players, rather than treating donated brains as a representative sample.
The finding is a population estimate for a defined group of deceased players, not a diagnosis of three out of four living NFL retirees. It also does not mean that every player with the disease’s brain changes experienced dementia or other symptoms. Those limits matter as families weigh a striking number against what scientists can establish about an individual.
According to Boston University’s account of the research, the team examined available death-certificate records from the National Death Index for 2,362 former NFL players. Those records helped researchers investigate how players whose brains were donated differed from players whose brains were not.
Among 387 deceased players whose donated brains were studied, 362 had some degree of CTE neuropathology, roughly 94%. The team considered factors associated with donation, including cognitive symptoms, age at death, playing position and years in the league. It estimated that families of former players with cognitive symptoms were about five times more likely to donate their brains than families of players without those symptoms.
That difference is the study’s methodological hinge. If a brain bank disproportionately receives donations from people who had symptoms, its observed disease rate cannot simply be assigned to everyone who played professional football. The researchers used the estimated selection ratio to adjust the donor finding downward to 74.5%.
The resulting number remains concerning, but it answers a more specific question than a count of positive autopsies alone. It attempts to estimate disease among players who never entered the brain bank, while depending on the information and assumptions used to model who became a donor. Those unexamined brains were not directly diagnosed.
Lead author Julia Gallini, a Boston University biostatistics PhD candidate, said in the university’s report that estimating prevalence helps define the scale of the problem and guide priorities for the medical needs of former players. She called for greater support for families, progress toward diagnosis during life and treatment of CTE-related symptoms.
The study appeared in JAMA Network Open. Boston University said the research was a collaboration with Mass General Brigham and the Concussion & CTE Foundation.
CTE is a neurodegenerative disease associated with repetitive head impacts. Research described by the university links it to changes in brain proteins and disrupted cell function, with possible manifestations including memory loss, impaired impulse control and movement problems. The presence of pathology and the experience of symptoms are related questions, but they are not interchangeable measures.
The distinction is especially important in early-stage disease. Boston University said not every player with CTE pathology goes on to develop symptoms. The new prevalence figure therefore should not be presented as a dementia rate, a measure of disability or evidence that a particular player’s behavior resulted from CTE.
The disease currently can be definitively diagnosed only after death. That leaves researchers trying to connect autopsy findings with what happened during a person’s life, while working toward a test that could identify the condition in living patients. It also limits what this study can tell a retired player who is worried about his own health today.
An earlier study from the same research group examined players who died between 2016 and 2021 and produced a much wider possible range of CTE prevalence. The new work does not mean that disease suddenly became more common between the two publications. It uses a different period and an adjustment for selection bias to address uncertainty about how representative donors were.
Former NFL quarterback Matt Hasselbeck, an ambassador for the university-led DIAGNOSE CTE study, framed the findings as a reason to participate in research rather than turn away from them. In the university’s account, he said, “I can’t take back the hits I took, but I can sign up for research and do whatever I can to help the scientists so they can find a cure in my lifetime.”
Senior author Yorghos Tripodis, a Boston University professor of biostatistics, acknowledged that the results could worry living players and their families. He said discoveries from the brain bank had moved researchers closer to diagnosis during life, with effective treatments remaining a central goal.
For now, the study sharpens an estimate without removing that diagnostic gap. Its central number reaches beyond donated brains through statistical adjustment; for a living former player, the question of whether those changes are present in his own brain remains unanswered.
