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A study in JAMA Network Open estimates that nearly 75% of about 2,360 former NFL players who died from 2008 to 2023 had signs of CTE. Because CTE can currently be diagnosed only after death and the estimate relies on statistical adjustments for brain-donation bias, the figure is not a direct count or a measure of how many players had symptoms.
A study published Wednesday in JAMA Network Open estimates that nearly 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 samples and statistical modeling—not an autopsy of every player—and the researchers say CTE cannot currently be diagnosed in living people.
The research team, led by Yorghos Tripodis of Boston University’s Alzheimer’s Disease and CTE Center, combined information from 396 donated brains in the UNITE Brain Bank with death-record data for another 1,966 former NFL players. The analysis covered about 2,360 deceased players in total. Signs of CTE were found in roughly 93% of the donated brains, but researchers adjusted their estimate because donations are more likely when a player had cognitive symptoms.
The team calculated that families of players with signs of cognitive decline were about five times more likely to donate a brain than families of players without such symptoms. After adjusting for that difference, researchers estimated that about 1,760 of the players in the study group had CTE. They also estimated that about 27% had dementia, a separate outcome that can have multiple causes. The dementia estimate does not mean that all those cases were caused by CTE.
The new estimate is higher than one-quarter, the figure the same research group reported in an August study of former NFL players who died over a five-year span. The researchers described the updated result as an estimate rather than a confirmed count. About 1% of brains donated to banks in the United States, Austria and Brazil have signs of CTE, the study authors said, but that comparison involves donated brains generally and is not a direct comparison of disease rates in the general population.
What the Estimate Says About NFL Risks
The finding adds to evidence that repeated head impacts are a serious concern in professional football, while also highlighting the limits of what researchers can currently measure. CTE is linked with structural changes in the brain, but its presence does not mean a person will develop cognitive decline, dementia or other symptoms. The study’s estimate of CTE prevalence and its separate estimate of dementia should not be treated as interchangeable.
Because diagnosis is available only after death, researchers cannot use this estimate to identify which living former players have CTE or to determine their individual risk. The results may guide further research into diagnosis and prevention, but the study does not establish that every player with CTE signs had symptoms or that CTE caused each case of dementia. The NFL said it continues efforts to reduce concussions and head impacts.
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How Researchers Adjusted Brain Donations
CTE is associated with repeated head impacts. When the head moves sharply during a collision, the brain can strike or twist inside the skull. Dr. Julian Bailes, a neurosurgeon who was not involved in the study, said repeated impacts can contribute to the buildup of tau proteins and damage that accumulates over time. Symptoms associated with CTE can include memory problems and changes in mood or behavior, but similar symptoms can occur with other conditions.
A central obstacle to measuring CTE is that its characteristic microscopic changes do not appear on standard brain scans; confirmation requires examining brain tissue after death. That limitation also shapes the research evidence: donated brains are not a random sample of all former players. A previous Boston University study found signs of CTE in 99% of donated brains from former NFL players, a result that has faced criticism because families may be more likely to donate when a player had symptoms. The latest analysis attempts to account for that selection bias rather than simply applying the donated-brain percentage to all deceased players.
““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 Prevalence Estimate
The estimate is subject to uncertainty because most players in the study did not have donated brains available for examination. Researchers used national death records, estimates of cognitive impairment and dementia, and assumptions about differences in donation rates to calculate prevalence. The result is therefore not a count of confirmed CTE diagnoses, and the study’s reported figures do not show how many players had symptoms attributable to CTE.
CTE can only be confirmed through examination of brain tissue after death, and signs of the disease do not always lead to noticeable symptoms. The study also does not establish that CTE caused the dementia estimated in 27% of the players; Tripodis said dementia can have various causes, while adding that CTE is likely to contribute in this particular population. The supplied report does not provide a confidence interval for the 75% estimate, so its statistical range cannot be stated here.
Prevention and Living Diagnosis Research
Researchers and clinicians have called for more work on ways to detect CTE during life and to understand which people with brain changes develop symptoms. Until a living diagnostic test is available, prevalence studies will continue to rely on postmortem brain examinations and estimates that account for selection bias. The study does not announce a timetable for a diagnostic test or a follow-up analysis.
Tripodis said reducing brain injuries and impacts is a key prevention goal. The NFL has cited measures including redesigned helmets, such as the Guardian Cap, and a dynamic kickoff rule intended to reduce collision speeds. The study did not assess those interventions or establish how much they reduce long-term CTE risk; their effects remain a separate question for future research.
Key Questions
Does the study confirm that 75% of former NFL players had CTE?
No. It estimates that nearly 75% of the former players who died from 2008 to 2023 had CTE signs. The estimate combines brain-donation data with statistical adjustments; most players in the group did not have a donated brain examined.
Can CTE be diagnosed in a living person?
According to the researchers and experts quoted in the report, CTE can currently be confirmed only by examining brain tissue after death. The study does not describe a test that can diagnose the condition in living people.
Does having CTE signs mean someone will develop dementia?
No. The report says some people with CTE signs may never develop symptoms. The study separately estimated that about 27% of the players had dementia, which can have multiple causes; that figure is not the same as the CTE estimate.
How did the researchers account for bias in donated brains?
The team estimated that families of players with cognitive decline were about five times more likely to donate a brain than families of players without those symptoms. It used that estimated difference in its statistical adjustment before calculating the overall CTE estimate.
Source: nbcnews.com
