About 75 Percent of NFL Players Who Died in 2008 to 2023 Had CTE, a Bias-Adjusted Estimate Suggests

Chronic traumatic encephalopathy, or CTE, has been found in the brains of former football players for years, but one question has stalled the debate: how common is it really? Most of what is known comes from donated brains, and players with symptoms were more likely to be donated. A study published on Wednesday tries to correct for that, and lands at roughly three in four.

The paper, "Bias-adjusted prevalence of chronic traumatic encephalopathy among deceased National Football League players", appears in JAMA Network Open and was released under embargo until 11 a.m. Eastern on 28 October, according to a Boston University School of Medicine release. It estimates that 74.5 percent of NFL players who died between 2008 and 2023 had CTE neuropathology.

How did the researchers get to 74.5 percent?

They started with 2,362 former NFL players who died in that period, identified through death certificate records in the National Death Index. Of 387 whose brains were examined, 362, or 94 percent, had some degree of CTE. The authors say that figure overstates the rate, because players with cognitive symptoms were about five times more likely to have their brains donated.

Using that ratio, they adjusted the donor figure down to an estimate for all 2,362. The JAMA Network summary notes selection bias inflates donor-based estimates, but "does not fully explain the high burden of CTE neuropathology" among these players. The authors add that CTE is found in less than 1 percent of donors to community brain banks.

What does the number not tell us?

Several things. The estimate covers deceased former players only; it says nothing about how many living or current players have CTE. Having the pathology does not mean someone had symptoms, and not everyone with it develops them. And the adjustment rests on an estimated selection ratio, which is the central uncertainty. Medical Xpress points out that the same group's August paper in The BMJ gave a much wider range, 25 to 98 percent, for players who died from 2016 to 2021.