Approximately 3 in 4 former National Football League players who died between 2008 and 2023 had chronic traumatic encephalopathy, according to a new study published Wednesday in the journal JAMA Network Open. The findings suggest an even higher prevalence than identified in certain previous analyses.
Chronic traumatic encephalopathy, or CTE, is a progressive neurodegenerative disease linked to repetitive head impacts commonly sustained in contact sports like football, soccer, and hockey. The condition has also been detected in military veterans exposed to traumatic explosions and victims of domestic violence. Brain trauma causes an abnormal buildup of the protein tau, and these cellular changes can begin years before any symptoms manifest.
While CTE can only be definitively diagnosed after death, it is associated with symptoms overlapping with dementia, including depression, anxiety, memory loss, and a lack of impulse control. Previous studies had detected the disease in up to 99% of former NFL players whose brains were donated for research. However, those findings carried a significant selection bias, as brains were typically donated by family members who noticed symptoms during the player’s lifetime. Conversely, a study published in August estimated that at least 1 in 4 former NFL players who died between 2016 and 2021 had CTE by comparing death certificates to confirmed brain donations.
To establish a clearer picture across a broader cohort, Dr. Yorghos Tripodis and researchers at Boston University’s CTE Center examined data from the Understanding Neurologic Injury and Traumatic Encephalopathy brain bank combined with National Death Index records. The study evaluated both brain donors and non-donors who died between January 2008 and December 2023.
Out of 2,362 players who died during that timeframe, 396 had donated their brains for research. Among those donors, 362—representing 93.5%—had been diagnosed with CTE. Nearly 70% of the total players studied died at age 70 or older.
By analyzing death certificates for all 2,362 deceased players, the research team discovered that former athletes who exhibited cognitive symptoms related to dementia were five times more likely to have their brains donated. Families were also more prone to donate if a player displayed more severe symptoms or died at a younger age. Researchers adjusted the disease rate among donors to account for these donation probabilities and applied those corrections to the wider population of former players.
Dr. Yorghos Tripodis noted limitations in the work, emphasizing that no study can completely eliminate selection bias, and the calculations cannot be extrapolated to populations outside former NFL players. He also stressed that physical evidence of CTE in the brain does not guarantee that every individual experienced symptoms.
“Our numbers suggest that the prevalence of dementia, which is the most severe type of cognitive symptoms, is four times higher than the general population among the NFL players,” Tripodis said. “The symptoms are high, but doesn’t mean that everybody will have symptoms, doesn’t even mean that everybody with symptoms will have the most severe symptoms at the end of their lives.”
External experts offered mixed perspectives on the methodology and implications. Dr. Rachel Grashow, a senior research scientist at Harvard University who was not involved in the study, pointed out that understanding the motivations behind brain donation remains tricky. She emphasized the importance of distinguishing CTE from dementia, noting that CTE involves tau protein aggregation measured in stages I through IV, with researchers finding higher dementia rates only in stages III and IV.
Dr. Julie Stamm, a clinical assistant professor at the University of Wisconsin-Madison who also did not participate in the research, expressed surprise at the high number of presumed CTE cases calculated after adjusting for donation bias. Stamm suggested the NFL could take more aggressive safety measures, such as eliminating tackling in practices at youth levels, which would help reduce cumulative head impacts over a lifetime.
When asked about the latest findings, the NFL directed attention to a statement issued following the August study, highlighting the league’s continuous efforts to make football safer, reduce concussions, and provide the football community with expanding physical and mental health resources.
Tripodis expressed hope that precise estimates of CTE cases in deceased players will drive increased investment into research, aiding the development of diagnostic criteria for living patients and clinical trials aimed at finding a cure.
