At least one in four National Football League players who died between 2016 and 2021 had signs of chronic traumatic encephalopathy, the degenerative brain disease linked to repeated blows to the head, according to a large new study that offers the most rigorous estimate yet of how widespread the condition really is in American football.
The research, led by scientists at Mass General Brigham and Boston University working with the Concussion & CTE Foundation, was published in The BMJ. It examined all 878 former NFL players known to have died during the six-year window, and the 235 families among them who chose to donate the player's brain for study.
Of those 235 donated brains, 215 — more than 91% — showed pathological evidence of CTE, a disease that can currently only be confirmed after death. Because families who suspect a loved one suffered are more likely to donate a brain in the first place, researchers treated that figure as skewed rather than representative of every player who died.
Instead they calculated a conservative floor: at least 24.5% of all 878 players who died in the period had CTE. The true prevalence, they say, could run as high as 97.7% — with the real number, in the words of lead author Dr. Daniel Daneshvar, "somewhere in the middle."
"To find that somewhere between 24.5% and 97.7% of NFL players from that time period had this progressive brain disease is a wake-up call for the NFL and scientific communities," said Daneshvar, chair of physical medicine and rehabilitation at Mass General Brigham and a Harvard Medical School associate professor.
The disease's toll went beyond the brain scans. Among the players diagnosed with CTE, 61.3% had met clinical criteria for dementia while they were alive, a share that rose above 90% among those with the most advanced, stage-four form of the disease.
Daneshvar's team stressed that NFL games are only part of the exposure. A significant share of the head impacts behind these cases, he said, were absorbed years earlier, in youth, high school and college football, long before any of the players signed an NFL contract.
CTE has haunted the league's image since a Pittsburgh pathologist first identified the disease in a former player's brain in the early 2000s, and again after the 2012 death of Junior Seau, the Hall of Fame linebacker whose posthumous diagnosis helped push the condition into the mainstream. Since then, the NFL has repeatedly overhauled its concussion protocols and playing rules in the name of safety.
Asked to respond to the new study, the league said it "continually works to make football safer," pointing to efforts to reduce concussions during games and expanded resources for former players' physical and mental health.
For the researchers, that is not enough on its own. "These data," Daneshvar said, "indicate that we need to dedicate resources towards developing a cure."
