Scientists estimated that about 3 in 4 former NFL players who died between 2008 and 2023 had CTE, after correcting for which families donate brains

Chronic traumatic encephalopathy, or CTE, is a brain disease linked to repeated hits to the head, and it can only be confirmed by examining the brain after death. Almost every NFL brain donated for study has shown it, but critics point out that families tend to donate when a player already had symptoms. Researchers at Boston University tried to correct for that. They combined brain bank results with national death records for all 2,362 former NFL players who died between 2008 and 2023.

After adjusting for which players were most likely to have their brains donated, the team estimated that about 74.5% of all former NFL players who died in that period had CTE. That is roughly three in four, and the highest estimate so far for the league as a whole.

Among the players whose brains were actually donated and examined, 93.5% had CTE. The new estimate is lower because it accounts for the fact that donors are not a random sample, but it is still far above anything seen in the general population.

The team also estimated how many of the players had dementia during their lives: about 27.2%, again after correcting for donation bias.

“No other environmental or occupational exposure ever studied is known to have increased dementia risk at death by that amount,” senior author Yorghos Tripodis, a professor of biostatistics at Boston University, said, according to news coverage of the study.

The question has hung over football research for years. In 2017, a widely cited case series in JAMA from the same Boston research program found CTE in 110 of 111 brains of former NFL players donated for study. Critics argued, fairly, that this mostly showed who gets donated, not how common the disease is across the league. The new study is an attempt to answer exactly that objection.

To do it, the researchers modeled how likely each player was to end up in the brain bank based on known characteristics, then reweighted the results to represent all 2,362 players who died. They repeated the analysis 1,000 times with different assumptions to see how stable the estimate was.

“The tricky part is understanding why someone donates their brain,” Rachel Grashow, a senior research scientist at Harvard University who was not involved, told CNN. Even with careful modeling, no study can fully remove that selection effect, and the authors acknowledge it.

The second finding is how much the estimate held up under different assumptions. The 95% uncertainty range around the main estimate ran from about 70% to 78%, and an alternative analysis using a different assumption about dementia gave 75.4%. Earlier work by some of the same researchers had put the minimum possible share for a shorter period at about 25%, the figure if every player who was not examined had been free of the disease, and the maximum near 98%.

For current and former players and their families, the findings strengthen the case that repeated head impacts carry long-term risks for the brain. Memory problems, mood changes or impulsive behavior in a former player are worth raising with a doctor, especially one familiar with head injury. Long-term brain health is shaped by many factors over a lifetime, from head impacts to sleep and daily rhythms.

There are limits to what this shows. The estimate depends on statistical assumptions about who donates, and those cannot be fully checked. CTE can only be diagnosed after death, the findings apply to NFL players rather than to athletes in general, and the study does not show how many head impacts, or what kind, lead to the disease. The NFL said in a statement responding to earlier research that it continuously works to make the game safer.

Source

Tripodis Y, et al. “Bias-Adjusted Prevalence of Chronic Traumatic Encephalopathy Among Deceased National Football League Players.” JAMA Network Open, 2026.
DOI: 10.1001/jamanetworkopen.2026.40884

Also cited

Mez J, Daneshvar DH, Kiernan PT, et al. “Clinicopathological Evaluation of Chronic Traumatic Encephalopathy in Players of American Football.” JAMA, 2017.
DOI: 10.1001/jama.2017.8334