Boston University Study Estimates 74.5% of Former NFL Players Had CTE; Experts Urge Caution and More Research

Boston University researchers examined the brains of 387 former NFL players who died between 2008 and 2023. Ninety-four percent of them had some form of chronic traumatic encephalopathy, or CTE. That number alone is stark, but it is not the one the study's authors chose to emphasize. They went on to estimate how common the disease was among all players who died in that window, and arrived at 74.5%.

CTE is a brain disease found in people who have taken repeated blows to the head. It can be definitively diagnosed only after death. It has turned up in first responders and military members as well as athletes, but it is rarely seen in the general population. The new work, published in JAMA Network Open, builds on data that appeared in The British Medical Journal in August. That earlier analysis produced a very wide range, running up to 98% of ex-NFL players who died between 2016 and 2021, and the width of the range reflected a real problem: no one knew how representative brain donors were of NFL players as a whole, since donors are more likely to have had symptoms thought to be linked to CTE.

The Boston team tried to correct for that. The 387 donors were a subset of the 2,362 former NFL players who died during the 15-year period. The researchers estimated that players with cognitive symptoms were five times more likely to have their brains donated to science than other players, and used that to adjust the figure downward from 94% to 74.5%.

Outside experts say even the adjusted number needs care. CBS News chief medical correspondent Dr. Jon LaPook cautioned that it may still be an overestimate. He noted that the study counted signs of CTE in the brain, not how many players actually showed symptoms of the disease.

Dr. Kristen Dams-O'Connor, director of the Brain Injury Research Center at Mount Sinai, said the findings cannot and should not be read as representative of current living athletes, pointing out that many of the players studied ended their careers long ago. The NFL has since changed rules meant to reduce concussions and introduced more protective equipment. She said that should mean lower CTE risk for recent generations compared with those who played decades ago. In a statement to CBS News, the league said it continuously strives to make the game safer, including through strategies to reduce concussions and head impacts.

Dams-O'Connor said Boston University used strong methods to adjust for selection bias, but gaps remain. One is accounting for roughly 20,000 NFL players who could have died during the study period but did not. She also said the results do not apply to the vast majority of people who played youth football or other non-professional contact sports.

Dr. Jeremy Tanner, an assistant professor of neurology at the Glenn Biggs Institute for Alzheimer's and Neurological Diseases at the UT San Antonio Health Science Center, called it a helpful look at the bigger picture. Even after accounting for selection bias, he said, the prevalence is at a minimum 20 to 25%, which he described as eye-opening and a sign of the need to advance research.

Dr. Jay Alberts, director of the Cleveland Clinic Concussion Center, argued the data point toward looking for early warning signs. Neurological disease is often described as having a silent phase, he said, but he does not think it is truly silent; researchers need to listen more carefully with better measurements. He suggested studying instrumental activities of daily living, such as driving or balancing a checkbook, which demand cognitive and motor function at once. In Alzheimer's and Parkinson's patients, those abilities begin to change five to seven years before diagnosis, and he said CTE may follow a similar pattern. Earlier detection could open the door to interventions that slow or even potentially reverse the disease.

Alberts and Tanner also pointed to research on biomarkers. Ongoing studies, including one with a site at UT Health San Antonio, follow current or former players across their lives to track changes in the body and brain. Tanner hopes tools developed there will let scientists detect CTE-related changes in living people, and ultimately find better ways to prevent or treat it.