Study finds at least 1 in 4 former NFL players had CTE at time of death

A new study published Tuesday in the medical journal The BMJ estimates that at least one in four former National Football League players who died between 2016 and 2021 had chronic traumatic encephalopathy, the degenerative brain disease linked to repeated head impacts, at the time of their death. The true rate could be far higher, the researchers say, because the disease can currently be diagnosed only after death, in a full autopsy of brain tissue.

The research was conducted by scientists at Mass General Brigham, Boston University and the Concussion Legacy Foundation, and was published Tuesday by Boston University's own research office. It is the first attempt of its kind to estimate CTE prevalence across the entire population of former players who died in a given period, rather than relying only on the smaller group of families who chose to donate a brain for study.

A wider, more rigorous estimate

Earlier research on football and CTE, including a widely cited 2017 study that found the disease in 110 of 111 donated NFL brains, drew criticism for relying on "convenience samples" — brains donated by families who often suspected something was wrong, a group that researchers acknowledged skewed toward more severe cases. The new study, titled "Prevalence of chronic traumatic encephalopathy at death in National Football League players" and published in The BMJ, was designed to correct for that bias by starting with a complete accounting of deaths rather than a pool of volunteers.

Researchers built a roster of every player who had appeared in at least one NFL game going back decades, then cross-referenced it against death records to identify former players who died between 2008 and 2021. Within that broader window, they focused on the six years from 2016 to 2021, a period in which 878 former players died and death-certificate data was most complete. Of those 878, families of 235 donated the player's brain for neuropathological examination; 215 of those brains, or roughly 92 percent, showed evidence of CTE.

Because researchers cannot know how many of the 643 players whose brains were not examined also had the disease, they calculated a range rather than a single figure: a minimum prevalence of 24.5 percent, assuming none of the undonated brains had CTE, and a maximum of 97.7 percent, assuming nearly all of them did. Across the full 2008-2021 study period, which included 1,712 former player deaths and 338 brain donations showing CTE in 93.2 percent of cases, the comparable range was 18.5 percent to 98.7 percent.

• 878 former NFL players died between 2016 and 2021, the study's core analysis window
• 235 of those players' brains were donated and examined; 215 showed CTE, a 91.5 percent rate among donors
• The population-wide estimate ranges from a minimum of 24.5 percent to a maximum of 97.7 percent
• 104 of the CTE-positive brains, or roughly 31 percent, showed the most advanced form, stage IV disease

Dementia and disease severity

The study also examined how CTE severity related to other health outcomes. Among brains with the disease, 59.8 percent came from players who had been diagnosed with dementia, and in 18.6 percent of cases dementia was listed as a primary cause of death. Players with stage IV CTE, the most advanced classification, were substantially more likely to have shown dementia symptoms before death than those with milder forms of the disease, according to the findings summarized in The BMJ's own publisher release, which described the paper as one of the most informative estimates to date of CTE burden among former NFL players.

The players in the study largely competed during the era of hard-shell helmets and before many of the modern safety protocols the league has since adopted, including limits on full-contact practices and an in-game concussion protocol. Researchers said that timing matters for interpreting the results: the estimate describes risk for a generation of players whose head-impact exposure came under rules that have since changed, not necessarily the risk facing players today.

Who is affected

The findings are most directly relevant to the tens of thousands of men who played in the NFL over the decades covered by the study, and to their families, since CTE has been associated with mood changes, memory loss, impulsivity and cognitive decline that can emerge years after a playing career ends. Because the disease cannot be diagnosed in a living person, players and relatives currently have no way to know with certainty whether symptoms they are experiencing stem from CTE, a treatable condition, or something else. Researchers involved in the study said that ambiguity is itself part of the problem the field is now trying to solve, noting that Boston University has diagnostic research projects underway aimed at eventually identifying the disease before death.

The NFL Players Association, which helps fund CTE-related research at Boston University and other institutions, said it sent an email to all players and their agents after the study's release, according to a summary of the union's message reported by NBC Sports. The union wrote that "the findings are a sobering reminder that football carries serious risks that can have lasting consequences for players and their families," and told members that "many of the symptoms associated with CTE are treatable with the right care," pointing to improvements it said some players have seen under specialists in neurodegenerative disease. The NFLPA said it would host medical calls for players over the following two weeks and pointed to its existing partnerships with the American Brain Foundation and Harvard University's Football Players Health Study.

Reaction and what happens next

Researchers on the study framed the findings as a call for more support and treatment options rather than a final word on football's risks. Dr. Jesse Mez, co-director of the BU CTE Center, said the scale of the numbers demands attention.

"If at least 25 percent of NFL players who died had these changes in their brain, then this seems like a call to action for more discussion about risk, more research, and more support for affected families."

Dr. Daniel Daneshvar of Mass General Brigham, one of the study's lead authors, said the wide range between the minimum and maximum estimates underscores how much remains unknown, and that the true figure "is somewhere in the middle," according to an account of the findings in the Boston Globe. He said the next phase of research needs to move beyond documenting the disease's prevalence and toward developing ways to treat and eventually cure it in people already affected. NPR's coverage of the study noted that no method currently exists to diagnose CTE in a living patient, which researchers said is the central obstacle to translating findings like these into care for people who need it now.

Boston University said it is continuing work on diagnostic research, including a longitudinal project intended to identify biomarkers of CTE in living former players, an effort that would let doctors intervene before symptoms progress. Absent such a tool, researchers and the players' union both said the near-term focus will be on managing symptoms — memory problems, mood changes and cognitive difficulties — that can overlap with CTE but, in some cases, stem from treatable conditions. The study's authors said they expect the findings to inform future consent and disclosure discussions for players entering the sport, as well as continued research funding decisions at the league and university level.