At least one in four former National Football League players who died between 2016 and 2021 had chronic traumatic encephalopathy at death, according to research published in The BMJ. The researchers describe 24.5 percent as a minimum, not a midpoint, and calculate that the true figure could run as high as 97.7 percent.
The study is the first serious attempt to place a population denominator under the CTE question. Previous estimates came almost entirely from donated brains, a group skewed toward families who already suspected the disease. This team, from Mass General Brigham, Boston University, and the Concussion and CTE Foundation, started instead with a roster of former players from the hard-shell helmet era.
The finding lands during football season, and its most immediate audience is not professional players. It is the parents of children who play tackle football at youth and high school level, and the families of former players now navigating dementia care.
Arithmetic That Produced the One in Four Figure
Researchers identified 1,712 former NFL players who died between 2008 and 2021, then narrowed to the years when brain donation was most frequent. Between 2016 and 2021, 878 former players died. Of those, 235 donated their brains, and pathologists confirmed CTE in 215.
That is 215 confirmed cases out of 878 deaths, or 24.5 percent. The calculation deliberately treats all 643 players whose brains were not examined as though none had the disease, which is almost certainly false.
The upper bound uses the opposite assumption. If every unexamined player had CTE except the 20 who were examined and cleared, prevalence reaches 97.7 percent. Lead author Dr. Daniel Daneshvar, chair of physical medicine and rehabilitation at Mass General Brigham, has said that both extremes are unlikely and that the real number lies in between.
Daneshvar has noted that community brain bank studies put CTE below 1 percent in the general population, and called the range the study produced "a wake-up call for the NFL and scientific communities."
Among the donated brains from that six-year window, 91.4 percent showed CTE. Across the full 2008 to 2021 period, 315 of 338 donated brains, or 93.2 percent, showed the disease. Dementia was common: 61.3 percent of donors with CTE met clinical criteria for dementia, and more severe CTE was more frequently associated with dementia.
Limitations That Belong at the Top, Not the Bottom
Several constraints shape how far this finding travels, and they are not buried in the paper.
CTE can only be confirmed by examining brain tissue after death. There is no approved test to diagnose it in a living person, so any claim that a current player has CTE is unprovable.
Brain donation is not random. Families often donate because they observed memory problems, mood changes or behavioral symptoms during life. That selection pressure is why the 91.4 percent and 93.2 percent donor figures cannot be applied to all players, and it is the problem this study was designed to work around rather than solve. Treating a donor rate as a population rate is the most common error in coverage of brain bank research.
The cohort also reflects an earlier era. These men played during the hard-shell helmet era, before more recent equipment changes and practice rules that limit contact. Whether current players face the same exposure remains untested, and the researchers say future work should examine athletes who played more recently.
The study establishes prevalence and its association with dementia. It does not prove any individual player's dementia was caused by CTE rather than Alzheimer's or another condition, and these diseases frequently coexist.
Exposure Starts Long Before the Professional Level
The point Daneshvar has emphasized most directly concerns children. Speaking to the Boston Globe, he noted that these athletes did not accumulate head-impact exposure only at the professional level but also in college, high school, and, in many cases, youth football, with all of those hits adding to the risk in their brains.
The mechanism researchers describe is cumulative rather than concussion-driven. Repeated subconcussive impacts, the routine collisions that never produce diagnosable symptoms, appear to matter more than the few hits that get a player pulled from a game. That changes what prevention looks like. Concussion protocols address visible events. They do not reduce total exposure.
The research team's stated conclusion is that prevention efforts should focus on reducing repetitive head impacts rather than concussions alone. Coauthor Dr. Jesse Mez of the Boston University CTE Center has noted that because the relationship between years of play and CTE risk is close to linear, higher prevalence might also be expected among college and high school players.
Reasonable Steps for Families This Season
Parents of young athletes have a practical question rather than a scientific one. The evidence supports reducing total head impacts rather than eliminating sport.
Flag football removes tackling entirely and is widely available through youth leagues. For families who continue with tackle football, the levers that reduce cumulative exposure are the number of full contact practices per week, coaching on technique, and the age at which contact begins. Parents can ask a league how many contact practices it holds, whether coaches complete certification training, and what its return-to-play protocol requires.
Anyone who played contact sports and is now experiencing memory problems, personality changes, worsening mood, or difficulty managing daily tasks should be evaluated by a neurologist. Those symptoms have many causes, several of them treatable, and should not be self-diagnosed as CTE. Families can seek evaluation through academic centers running dedicated clinical programs, and research on diagnosing CTE during life is active but not yet clinically available.
Whether the NFL adopts further practice restrictions, and whether youth participation shifts, are the developments to watch.
If you or someone in your household is struggling emotionally after reading about this research, support is available. Talk with a clinician, or contact the 988 Suicide and Crisis Lifeline by calling or texting 988.
Key Questions Answered
What did the study find? Of 878 former NFL players who died between 2016 and 2021, 235 donated their brains, and 215 were confirmed to have CTE. That produces a minimum prevalence of 24.5 percent, with an upper bound of 97.7 percent.
Why is the range so wide? Because 643 of the 878 players did not donate their brains, and CTE cannot be diagnosed without examining brain tissue. The low estimate assumes none of them had it. The high estimate assumes nearly all of them did. Neither extreme is likely.
Can CTE be diagnosed in a living person? No. Confirmation currently requires examination of brain tissue after death. There is no approved diagnostic test for living patients, though research toward one is active.
How common is CTE outside contact sports? Community brain bank studies cited by the researchers put prevalence below 1 percent in the general population. The disease is strongly associated with repeated head impacts.
Does this mean my child should not play football? The study does not answer that. It examined former professional players from an earlier era of equipment. Researchers recommend reducing cumulative head-impact exposure, which can mean playing flag football, having fewer full-contact practices, or delaying the age at which tackling begins.
What symptoms should a former player watch for? Memory loss, confusion, mood changes, impulsive behavior, and difficulty with daily tasks. These symptoms have many possible causes, several of them treatable, so evaluation by a neurologist is the appropriate step rather than assuming CTE.
What happens next? Researchers have called for continued work on diagnosing CTE during life, for studying athletes who played more recently, and for prevention aimed at repetitive head impacts rather than concussions alone. Whether the NFL or youth leagues change contact rules is the next thing to watch.
