Study Estimates Three in Four Former NFL Players Who Died Over 15 Years Had CTE

An estimated 75% of professional football players who died over a 15-year period showed signs of chronic traumatic encephalopathy, or CTE, according to a study published Wednesday in JAMA Network Open. It is the highest estimate to date.

The same research group published a study in August that estimated one-quarter of NFL players who died over a five-year span had CTE. The new figure is three times that number. A smaller share of the former players, about 27%, were estimated to have dementia. Not everyone with signs of CTE in the brain develops cognitive decline or dementia, and some never have symptoms.

Study leader Yorghos Tripodis, director of statistics and data management at the Boston University Alzheimer's Disease and CTE Center, said the result surprised him. "I had to double-check the numbers," he said. He added that the figure is a best guess, because "right now we don't know how to diagnose CTE when people are alive, so we are trying to estimate." The center houses the UNITE Brain Bank, which studies the effects of repetitive head trauma in donated brains.

A major obstacle in measuring how common CTE is lies in the fact that its microscopic changes do not appear on brain scans, so it can be diagnosed only at autopsy. Past research has also been criticized for bias, since people with cognitive decline and their families are much likelier to donate brains or request autopsies. An earlier Boston University study found that 99% of brains donated by former NFL players showed signs of CTE.

To limit that bias, Tripodis and colleagues combined UNITE Brain Bank data with national death records for other former NFL players who died from 2008 to 2023. They examined 396 donated brains and information on 1,966 other deceased players, and factored in ages at death and estimated rates of mild cognitive impairment and dementia among former players older than 50.

About 93% of the donated brains showed signs of CTE. The team calculated that families of players with symptoms of cognitive decline were about five times likelier to donate brains than families of players without symptoms, and used a statistical analysis to correct for that. They estimated that about 1,760 of the roughly 2,360 players who died in the period, or nearly 75%, had CTE. By comparison, only about 1% of brains donated to banks in the U.S., Austria and Brazil show signs of the disease, the researchers said.

Outside experts urged caution. Dr. Jim Kelly, a professor emeritus of neurology at the University of Colorado Anschutz who was not involved in the research, said: "These are all estimates. They had no autopsies on five times as many players who died as they did have autopsies for." He noted that some autopsies may have revealed only one or a couple of lesions linked to CTE, which seems unlikely to lead to behavior change or dementia.

Dr. Julian Bailes, chair of neurosurgery at Endeavor Health in the Chicago area, who also was not involved, explained that repeated head impacts can cause tau proteins to build up in the brain. The disease can affect areas that regulate behavior and emotions, leading to memory loss, depression and violent mood swings, though those symptoms can stem from other illnesses. He said 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 said prevention is key: "If you reduce the number of brain injuries, brain impacts, you substantially decrease the 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 redesigned helmets such as the Guardian Cap and the dynamic kickoff rule, which leads to lower collision speeds.