Study details and key findings

CTE is believed to cause deterioration of brain matter and the abnormal accumulation of the protein tau in the brain. Currently, CTE can only be diagnosed post-mortem by examining the brain tissue of deceased subjects. However, scientists believe that people diagnosed with the disease post-mortem can present with symptoms while they're living. Some potential indicators of the disease include aggression, anxiety, confusion, dementia, depression, memory loss, and substance misuse. There is no known treatment for the disease.

For the study, researchers created a list of every player who had appeared in at least one NFL game since 1949 and then referenced demographic data and obtained death certificates from CDC to create a list of 1,712 players who died in that time period.

They then narrowed the sample to those who had died between 2016 and 2021, when the proportion of players whose brains had been donated for study was the highest. Ultimately, of the 878 former players whose brains the researchers analyzed, at least 215 — or 24.5% — had CTE.

According to the study's lead author Daniel Daneshvar, chief of brain injury rehabilitation at Spaulding Rehabilitation Hospital and an associate professor at Harvard Medical School, the estimate of 25% is on the "low end." At the upper end, the maximum possible prevalence of CTE was around 97%, but the true rate "is somewhere in the middle," Daneshvar said.

The study also looked at the connection between CTE and dementia and found that more than 90% of samples of Stage 4 CTE, which is the most advanced stage of the disease, were also diagnosed with dementia.

Discussion

Randy Grimes, who played center for the Tampa Bay Buccaneers between 1983 and 1992, said he'd bet "the real number [of football players with CTE] is a lot bigger" than 25%.

According to Grimes, he hasn't yet felt the symptoms that would make him worry about his own cognitive health, but he runs a nonprofit that counsels retired athletes, including football players, and sees symptoms he suspects are associated with CTE.

In the future, the study's authors hope to produce another paper estimating the overall rate of CTE in all NFL players at the time of their death. Although making such an estimation will likely be complicated by a variety of unknowns, the researchers said one thing is certain — CTE is preventable.

"We know what's causing it," Daneshvar said. "It's a neurodegenerative disease where we know and have complete control over the risk. And understanding the extent of that risk is one of the steps into making sure that we can mitigate and decrease that risk."

Daneshvar said the biggest predictor of CTE is the cumulative force of hits to a head over a lifetime, including hits sustained before reaching the NFL.

As the scientific understanding of CTE and its relationship to head trauma has grown, the NFL has taken steps to improve player safety, including banning certain dangerous plays like traditional kickoffs and helmet-to-helmet tackles, and funding research of safer equipment.

"It was captured during a time when there was delayed recognition by the NFL that concussion was a big deal," Chesnut said. "So this may not represent the NFL in 2026."

In a statement, the NFL Players Association (NFLPA) said the study is "a sobering reminder that football carries real risks, and those risks can have lasting consequences for players and their families" and that it "reinforces one of the NFLPA's core priorities: improving player safety and supporting players throughout their lives, both during and after their careers."

Similarly, the NFL said that it "continuously strives to make the game of football safer, including by implementing strategies to reduce concussions and head impacts. The NFL remains committed to ensuring that the NFL community has access to a robust — and expanding — set of resources to enhance their physical and mental well-being."