A new study says chronic traumatic encephalopathy may be far more common among former NFL players than many people realized. Researchers estimate that about 75% of former NFL players who died between 2008 and 2023 had CTE, the brain disease linked to repeated head impacts.

The study was published Wednesday in JAMA Network Open by researchers at Boston University, Mass General Brigham and the Concussion & CTE Foundation. The authors called it the first comprehensive effort to measure how widespread CTE is among deceased former NFL players.

CTE can only be diagnosed after death. Scientists are still working on a test that could detect it in living people.

How the estimate was built

Past CTE research has faced criticism because it relied on brains donated by families of players who often had symptoms. This study tried to correct for that. It looked at 2,362 cases, combining 396 players whose brains were donated with death records for 1,966 players whose brains were not.

Nearly 94% of the donated brains showed CTE. Donors were more likely to be white, to have died younger and to have played more seasons. Even after adjusting for those differences the researchers said the rate stayed high.

Lead author Julia Gallini said the nearly 75% estimate was "concerning." She said more needs to be done to support families, to diagnose CTE in living people and to treat its symptoms.

What the numbers do not say

The researchers stressed that the estimate applies only to former players who have died. It does not predict how many living NFL players have CTE. It also does not cover college or youth football.

The disease is graded in four stages. The early stages often cause no symptoms. "Many cases may have been early stage at the time of death and possibly symptom free," the paper said.

The league responds

The NFL said it "continuously strives to make the game of football safer, including by implementing strategies to reduce concussions and head impacts." The league encouraged former players to use its health resources if they are worried about their health.

The NFL Players Association said studies like this one "are critical to improving our understanding of football's long-term impacts."