Study estimates CTE in about three-quarters of former NFL players who died from 2008 to 2023

That donor group was not representative of all the players studied. Researchers estimated that people with cognitive symptoms were about five times more likely to donate their brains, then adjusted the overall estimate for that difference, Scientific American reports. CTE diagnosis requires examination of the brain after death, so the estimate for players who did not donate is not based on individual brain examinations.

What the figure does not show

The researchers cautioned that the estimate does not predict CTE prevalence among living NFL players or address college and youth players, ESPN reports. ESPN also quotes the study as saying many cases may have been early-stage at death and possibly symptom-free. Russell Lonser, a neurological surgery professor at Ohio State University who was not involved in the study, told Scientific American that the figure is an estimate from a model and includes early changes that may never have caused symptoms.