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Researchers estimate that about 75% of former NFL players who died between 2008 and 2023 had signs of chronic traumatic encephalopathy, or CTE. The estimate uses donated-brain findings and statistical adjustments for donation bias; CTE can currently be confirmed only after death, and the estimate does not mean that 75% had dementia or symptoms.
A study published Wednesday in JAMA Network Open estimates that about 75% of former NFL players who died from 2008 to 2023 had signs of chronic traumatic encephalopathy, or CTE. The estimate is based on donated-brain data adjusted for potential donation bias, not an examination of every player in the group; CTE can currently be diagnosed only after death.
The Boston University research team combined findings from 396 donated brains in the UNITE Brain Bank with death-record information for another 1,966 deceased former NFL players. Researchers used statistical methods to account for the possibility that families of players with cognitive symptoms were more likely to donate a brain. Based on the combined data, they estimated that about 1,760 of roughly 2,360 players who died during the 15-year period had CTE.
About 93% of the donated brains showed signs of CTE. That figure describes the brain-bank sample, not all former NFL players. The researchers estimated the prevalence across the larger group after accounting for selection bias and other factors, including age at death and estimated rates of mild cognitive impairment and dementia among former players over 50.
The team also estimated that about 27% of the players in the analysis had dementia. That is a separate estimate: dementia can have multiple causes, and the study does not establish that CTE caused every case. Researchers also stress that CTE signs do not necessarily lead to cognitive decline, dementia or any noticeable symptoms.
What the Estimate Says About Risk
The findings add to evidence about the possible long-term effects of repeated head impacts in professional football. They may inform efforts to reduce those impacts and support further research into diagnosis and treatment. But the estimate should not be read as a direct measurement of CTE among all living or deceased NFL players: it is a statistical estimate for a defined group of former players who died during 2008–2023.
The distinction between brain signs and symptoms matters for families, players and clinicians. CTE is identified through structural changes seen in brain tissue after death, while problems such as memory loss, depression and changes in behavior can have other explanations. A high estimated rate of brain pathology does not show how many players experienced symptoms during life, or whether those symptoms were caused by CTE.
Researchers and outside experts say the lack of a reliable way to diagnose CTE during life limits what doctors can do for people who may be affected. It also makes prevalence research dependent on donated brains and statistical methods. The study’s estimate can guide questions for future research, but it is not a clinical test or a prediction for an individual player.
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How Researchers Adjusted Brain Donations
Earlier research based on donated brains has faced a central limitation: families may be more likely to arrange a donation or autopsy when a player had cognitive or behavioral difficulties. A Boston University study found signs of CTE in 99% of donated brains from former NFL players, but that selected group cannot by itself establish how common CTE is among all former players. The new analysis attempted to estimate and correct for that selection effect.
The new estimate is higher than an earlier estimate from the same research group, which reported that roughly one-quarter of NFL players who died during a five-year span had CTE. The current study covers a longer period and applies its analysis to a larger set of death records. It is not a comparison of two directly identical samples, so the change should not be treated as proof that CTE prevalence rose over time.
CTE is associated with repeated head impacts. Dr. Julian Bailes, a neurosurgeon who was not involved in the study, described how repeated impacts may allow tau proteins to accumulate and damage the brain. The condition’s microscopic changes do not appear on routine brain scans, according to the report, leaving examination of brain tissue after death as the way to confirm it at present.
“Right now we don’t know how to diagnose CTE when people are alive, so we are trying to estimate.”
— Yorghos Tripodis
Limits of the CTE Estimate
The figure is an estimate, not a count of confirmed diagnoses across all 2,360 players. Only 396 brains were examined, and researchers corrected for the likelihood that families of players with symptoms were more likely to donate. The study’s result therefore depends on the data and assumptions used in that adjustment. The source material does not provide a confidence interval for the estimate.
CTE cannot currently be confirmed while a person is alive, and the study cannot establish which players had symptoms or whether CTE caused them. Some brains may show only a small number of lesions; outside expert Jim Kelly said such findings may not lead to behavioral changes or dementia. The 27% dementia estimate is not an estimate of dementia caused solely by CTE.
It is also unclear from the reported findings how the estimate applies to players who are alive, players in other football levels, or people exposed to repeated head impacts outside professional football. The study describes deceased former NFL players in a particular 15-year period; it does not measure the risk for an individual athlete.
Diagnosis and Impact Prevention
The researchers say better ways to identify CTE in living people are needed so doctors can study its course and offer appropriate care. Until a reliable diagnostic method is available, future estimates will continue to depend heavily on brain donations, autopsy findings and statistical corrections.
In the meantime, study leader Tripodis said reducing brain injuries and impacts could lower risk. The NFL has introduced measures intended to reduce high-impact collisions, including redesigned helmets and changes to kickoffs, according to the report. The supplied information does not establish how much those measures have reduced impacts or whether they change long-term CTE rates.
Further research will be needed to test how well the new estimate holds up and to clarify how brain changes relate to symptoms over time. For now, the study offers a broader estimate than brain-donation figures alone, while leaving major questions about diagnosis, individual risk and prevention unresolved.
Key Questions
What did the study estimate?
Researchers estimated that about 75% of roughly 2,360 former NFL players who died from 2008 to 2023 had signs of CTE. This is a statistical estimate, not a confirmed diagnosis for each player.
Does having signs of CTE mean a player had dementia?
No. The researchers said some people with CTE signs may never develop symptoms. They separately estimated that about 27% of the players in the analysis had dementia, which can have different causes.
How was the estimate calculated?
The team analyzed 396 donated brains and death-record information for another 1,966 former players, then used statistical methods to account for higher donation likelihood among families of players with cognitive symptoms.
Can doctors diagnose CTE in a living person?
Not at present. The report says CTE can currently be confirmed only by examining brain tissue after death, which is why prevalence figures rely on donated brains and estimates.
Does the study show that recent NFL safety measures prevent CTE?
No. The study estimated CTE among players who died between 2008 and 2023. The report notes league measures intended to reduce high-impact hits, but the findings do not establish whether those measures have reduced long-term CTE rates.
Source: nbcnews.com
