Boston University Researchers Calculate 75% CTE Rate in Deceased NFL Athletes

An estimated 75% of former National Football League players who died between 2008 and 2023 showed signs of chronic traumatic encephalopathy, according to a study published Wednesday in JAMA Network Open. This figure represents the highest rate recorded to date and is three times higher than an estimate of 25% produced by the same research group in August for a different five-year period. The study also estimated that roughly 27% of the deceased players had dementia, though not all individuals with CTE markers in their brains developed cognitive decline.

Yorghos Tripodis, the study leader and director of statistics and data management at the Boston University Alzheimer's Disease and CTE Center, stated that the result was unexpected. "I had to double-check the numbers," he said. He noted that the figure is a best guess because there is currently no way to diagnose CTE in living patients. The center houses the UNITE Brain Bank, which studies the effects of repetitive head trauma in donated brains. The research team combined data from the brain bank with national death records to create a more comprehensive picture of the condition's prevalence.

Diagnosing CTE remains difficult because the microscopic changes in the brain do not appear on standard scans, meaning the condition can only be confirmed through autopsy. Past studies have faced criticism for selection bias, as families of players with cognitive symptoms are more likely to donate brains. An earlier Boston University study found that 99% of brains donated by former NFL players showed signs of CTE. To address this skew, the new research incorporated information on 1,966 deceased players who did not have their brains donated, alongside the 396 donated brains.

The researchers factored in ages at death and estimated rates of mild cognitive impairment among former players over the age of 50. They calculated that families of players with symptoms were about five times more likely to donate brains than those without symptoms. Using statistical analysis to correct for this bias, the team estimated that approximately 1,760 of the 2,360 players who died during the 15-year period had CTE. By comparison, the researchers noted that only about 1% of brains donated to banks in the U.S., Austria, and Brazil show signs of the disease.

Outside experts advised caution regarding the findings. Dr. Jim Kelly, a professor emeritus of neurology at the University of Colorado Anschutz who was not involved in the study, emphasized that the results are estimates. "They had no autopsies on five times as many players who died as they did have autopsies for," Kelly said. He also pointed out that some autopsies may have revealed only a small number of lesions, which might not be enough to cause behavioral changes or dementia.

Dr. Julian Bailes, chair of neurosurgery at Endeavor Health, explained that repeated head impacts cause tau proteins to build up in the brain. This buildup can affect areas regulating behavior and emotions, leading to memory loss, depression, and mood swings. Bailes stated that the findings call for more research into diagnosing CTE in living people. "If you can't tell if someone has a certain disease until they're dead, you really can't help them," he said.

Tripodis emphasized that reducing the number of brain injuries and impacts is the key to lowering risk. In an emailed statement, an NFL spokesperson said the league "continuously strives to make the game of football safer, including by implementing strategies to reduce concussions and head impacts." The league has introduced measures such as the Guardian Cap helmet and a dynamic kickoff rule designed to lower collision speeds.