3 in 4 NFL Players Who Died Since 2008 May Have Had the Brain Disease CTE

American football is a high-impact sport, and awareness is growing of the health risks posed by repeated impacts, especially when it comes to chronic traumatic encephalopathy (CTE), a brain disease linked to repeated hits to the head.

Scientists who’ve examined the donated remains of former NFL players have found evidence of CTE in nearly every case. But researchers have been quick to point out the selection bias that comes with these donations: Families of players who had symptoms of CTE before they died are more likely to donate a loved one’s brain, which can make the disease look more common than it really is.

CTE Is Linked to Years of Hits to the Head

CTE is a brain disease that gets worse over time, and it can be diagnosed definitively only by examining brain tissue after death, says the senior author Yorghos Tripodis, PhD, a professor of biostatistics at the Boston University School of Public Health.

Not everyone with CTE develops symptoms, especially when the disease is at an early stage. When symptoms do appear, they commonly include problems with thinking and memory, changes in behavior, and dementia.

“The only established risk factor is repetitive head impacts,” says Dr. Tripodis. Football players sustain a very large number of them over a career, and most never cause noticeable symptoms, he says.

Families of Players With CTE Symptoms Were About 5 Times as Likely to Donate

For the new study, researchers started with more than 375 donated brains that had a clear result. Among these, 94 percent showed evidence of CTE. Then scientists looked at national death records for all 2,362 former NFL players who died between 2008 and 2023, to compare players whose brains were donated with players whose brains weren’t.

Players who had dementia or other problems with memory and thinking before they died were about five times as likely to have their brains donated. Because of that, the researchers lowered the 94 percent figure to estimate how common CTE was among all the players who died, not just the donors.

After that adjustment, the researchers estimated:

• About 3 in 4 (74.5 percent) of the former players who died during those years had CTE.
• Even under much stricter assumptions about brain donation, the estimate stayed above 60 percent.
• About 27 percent of all former players had dementia during their lives, which the authors say is likely an underestimate.

The new findings suggest that studies of donated brains do overstate how common CTE is among NFL players, but not by enough to explain away the high rates, the authors wrote.

NFL Safety Changes May Reduce Concussions, but Not the Number of Hits

The National Football League has made a number of changes over the years to reduce concussions, including rule changes, penalties for certain harmful hits, and better protective equipment, says James Kelly, MD, a professor emeritus of neurology at the University of Colorado Anschutz in Aurora and the chief medical scientist for the Invisible Wounds Foundation, who wasn’t involved in the study.

Changes include:

• Helmet Rules Players are penalized for lowering their head to make forceful contact with their helmet, or for using any part of the helmet or face mask to hit an opponent’s head or neck.

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• Guardian Caps These soft, padded shells fit over the helmet to lessen the force of hits.

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   Players in positions with the most head contact must wear them during full-contact practices.

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• A New Kickoff In 2024, the league changed how kickoffs work, lining up players closer together and keeping most of them still until the ball is caught or hits the ground, to reduce injuries on the play. The concussion rate on kickoffs has remained below the rate observed under the old format, according to the league.

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• Better Helmets More than 98 percent of players now wear helmets that rank the highest in league lab testing for reducing the force of blows to the head, compared with about 70 percent before the 2025 season began.

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But concussions (head injuries that cause symptoms) aren’t the best measure of CTE risk, says Tripodis. Instead, CTE is linked to the total number of hits over time, including the many that never cause a concussion, he says.

“Equipment such as Guardian Caps may lessen the force of some impacts, but it does not reduce how many hits a player takes,” he says. Policies that limit full-contact practice at all levels of play could substantially reduce the risk, he says.

The evidence that Guardian Caps protect players is controversial, says Dr. Kelly. Even the NFL’s own data suggests that a drop in concussions couldn’t be attributed to the caps alone, he says.

About 4 in 10 of the players in the new study were 80 or older when they died, so many likely played long before the league’s recent safety changes.

Strengths and Limitations of the Study

The estimates held up across many different assumptions about who donates, the authors wrote.

Still, the researchers had to rely on stand-in metrics to judge which non-donor players had symptoms, including whether dementia was listed on death certificates, because it's often left off.

The study’s results are a statistical estimate for players whose brains were never examined, not a direct count, says Kelly. “So it’s really an elaborate method of estimation,” he says. He also notes that the study didn’t report other brain diseases found at autopsy that can cause dementia, such as Alzheimer’s disease.

“I’m not sure this study contributes significantly to our understanding of the problem,” he says.

Other factors that influence brain donation are harder to measure, says Rachel Grashow, PhD, a senior research scientist in environmental health at the Harvard T.H. Chan School of Public Health in Boston, who studies living former NFL players and wasn’t involved in the study. For example, players who died by suicide may have hidden or downplayed depression symptoms around their families, she says.

The results also can’t be applied to living former players. “Those who died earlier who had CTE may have been more likely to die due to CTE-related diseases, which may not be reflective of those who will still live for many years,” says Dr. Grashow.

Researchers Want Fans, Players, and Parents to Understand the Risks

“We share these findings so that current and future players understand the risks and can work together to make the game safer,” says Tripodis.

Because the risk rises with years of play, delaying the start of tackle football until an older age reduces the total number of hits a child takes, which could lower their risk of CTE, he says.

Tripodis also wants former players to know there are reasons for hope. Many of the players with CTE at death likely had early-stage disease, and not all had symptoms, he says. “We are making significant progress in diagnosing the disease in living people,” he says.

Grashow and her team are currently studying about 5,000 living former NFL players through Harvard’s Football Players Health Study. They have found that head injuries raise the risk of several treatable conditions, including sleep apnea, low testosterone, high blood pressure, depression, anxiety, and ringing in the ears. Left untreated, these conditions can harm the heart and brain, worsen memory and thinking problems, and raise the risk of dementia, she says.

Because these conditions themselves can also cause cognitive issues, former players with symptoms should check with a healthcare provider about anything treatable before assuming they have CTE, says Grashow.

Her research has also found that former players are especially vulnerable to depression and suicide, so framing brain disease as inevitable can do real harm, she says.