A landmark study published in the British Medical Journal has found that chronic traumatic encephalopathy (CTE) — a degenerative brain disease linked to repeated blows to the head — affected at least 24.5% of former National Football League players who died between 2016 and 2021, with researchers warning the true figure could be far higher. The study, led by Daniel Daneshvar, an associate professor at Harvard Medical School, examined the cases of 878 former players who died during that six-year period. Of the 235 who had donated their brains to research, 215 tested positive for CTE — a rate exceeding 90% within that sample. Researchers calculated a conservative minimum prevalence of 24.5% across the broader group, rising to a potential maximum of 97.7%.

CTE can only be definitively diagnosed after death through examination of brain tissue, which makes establishing true prevalence in living populations difficult. The disease, thought to be caused by years of sustained head trauma, can produce symptoms including depression, anxiety, aggression, and — in later stages — memory loss and dementia. The study found that nearly two-thirds of the brain donors had exhibited signs of dementia before they died, suggesting that for many, the disease had reached an advanced stage.

Dr. Chris Nowinski, co-founder and CEO of the Concussion and CTE Foundation and a co-author of the study, described the findings as