New Study: At Least 1 in 4 Former NFL Players Had CTE at Death, Some Estimates Run to Nearly 98%

A brain disease that can't be diagnosed until you're dead just got a harder number attached to it. At least one in four former NFL players who died between 2016 and 2021 had chronic traumatic encephalopathy, according to a study published Tuesday in The BMJ.

The Numbers

The study comes from Mass General Brigham, Boston University and the Concussion & CTE Foundation, three of the groups leading CTE research in the country. Dr. Daniel Daneshvar, chair of Physical Medicine and Rehabilitation at Mass General Brigham and a Harvard Medical School associate professor, led the work.

Researchers tracked 1,712 former NFL players who died between 2008 and 2021, all of whom had played at least one NFL game after 1949, when plastic helmets became mandatory. Only 338 of those players, about 20%, had their brains donated for study. Of those, 315 came back positive for CTE, a 93.2% rate among donors, according to News Medical's rundown of the findings.

Narrow the window to players who died between 2016 and 2021, when brain donation was most common, and 235 of 878 total deaths resulted in donated brains. The Guardian reports 215 of those 235 donated brains showed CTE.

Because donation is voluntary and skews toward families who noticed symptoms, the researchers built a statistical range instead of a single number. For the 2016-2021 group, that range runs from a minimum of 24.5% up to a modeled maximum of 97.7%, per News Medical. For the full 2008-2021 group, the range is 18.5% to 98.7%.

Daneshvar's team also found that players with the most advanced disease, stage IV CTE, were 44% more likely to have been diagnosed with dementia by a study clinician before death. Overall, 202 of the 338 donors, nearly 60%, had dementia diagnosed during their lives, and 63 had neurodegenerative disease listed as the actual cause of death.

Why the Range Is So Wide

CTE can't be diagnosed in a living person. Post-mortem brain tissue analysis is the only way to confirm it, which means every estimate of prevalence has to account for who chooses to donate and who doesn't.

Daneshvar's team was direct about that limitation. This is a retrospective, observational study built on voluntary brain donation, not a random sample of every player who ever suited up. Families who saw memory loss, mood swings or cognitive decline in a loved one are more likely to donate that brain for study than families who didn't. That selection bias is exactly why the researchers gave a range instead of a flat percentage, and it's a limitation they flagged themselves rather than one uncovered by critics.

Daneshvar told CBC News the team was "surprised but saddened to see that the minimum rate was so alarmingly high." A 24.5% floor, even accounting for selection bias, is not a trivial number for a disease this severe.

Where the Damage Actually Happens

One point Daneshvar made deserves attention: most of the head trauma driving these outcomes doesn't come from the NFL itself.

"The bulk of the head impacts that these former NFL players experienced didn't occur at the NFL level," Daneshvar told The Guardian. "They occurred at the collegiate level ... and at the high school level and in many cases the youth level. And all of those cumulative hits to the head added up to result in an increased risk."

That reframes the policy conversation. The NFL has made changes, cutting contact practices to less than one per week under its 2011 Collective Bargaining Agreement, and the Ivy League banned full-contact tackling in practice five years later. But if the cumulative damage starts in Pop Warner leagues and high school programs, NFL-level reforms only address part of the exposure.

A Penn State researcher, Dr. Reuben Kraft, is running an instrumented mouthguard study at Aliquippa High School in Pennsylvania, funded by the Chuck Noll Foundation, according to CBS News. Two Aliquippa players tracked last season averaged 162 head impacts across the year, comparable to Division I college numbers, largely because they played both offense and defense. Aliquippa athletic trainer Mike McGarvey said that double duty is "one of the big things that a lot of people miss."

The NFL's Response

The NFL, in a statement to The Guardian, said it "continuously strives to make the game of football safer, including by implementing strategies to reduce concussions and head impacts," and pointed to resources available to former players for their physical and mental health.

The league did not dispute the study's methodology or findings in that statement, and no NFL official challenged the specific percentages reported.

What's Still Unknown

CTE can't be diagnosed while someone's alive, which means every retired player currently living with symptoms, or without them, exists outside this data. Researchers like Daneshvar are among those working toward living diagnosis, but that tool doesn't exist yet.

The unresolved question is what changes at the youth and high school level, where researchers say the bulk of cumulative head trauma begins, well before anyone reaches an NFL roster.