At Least 25% of Former NFL Players Had Degenerative Brain Disease, Study Finds

The largest study ever conducted on CTE prevalence among deceased NFL players landed Monday in the British Medical Journal — and the findings are as disturbing as they are clarifying.

At least 25% of former NFL players who died between 2016 and 2021 had chronic traumatic encephalopathy at the time of their death, according to researchers from Harvard Medical School, Massachusetts General Hospital, Boston University, and other institutions. That 25% is the floor. The ceiling, based on the study's statistical modeling, could be as high as 97.7%.

The reason for that wide range comes down to how brain donation works — and why it creates an inherently skewed sample.

What the Study Actually Did

Researchers identified 1,712 former NFL players who died between 2016 and 2021 — meaning anyone who had played at least one NFL game since 1949. Of those, 235 brains were donated for research. Of those 235, 215 — more than 91% — showed signs of CTE.

The problem with that number, as the researchers are careful to explain, is selection bias. People who donate their brains to CTE research are disproportionately those who showed symptoms during their lifetime — memory loss, mood swings, impulse control problems — that made their families or the players themselves suspect something was wrong. The brains most likely to have CTE are the most likely to end up in the donation pool.

That means 91% isn't the prevalence rate across all former NFL players. It's the rate among the subset who had reason to think they might have it. The true overall rate sits somewhere between that floor of 25% and the ceiling of 97.7% — a range that reflects genuine statistical uncertainty rather than scientific imprecision.

"The prevalence estimate of 25% is at the low end," said lead researcher Dr. Daniel Daneshvar of Harvard Medical School and Massachusetts General Hospital.

What CTE Actually Does

Chronic traumatic encephalopathy is a neurodegenerative disease — meaning it progressively destroys brain tissue over time. It's caused by repeated blows to the head, not just concussions. The cumulative effect of thousands of smaller impacts over years and decades of play appears to be the primary driver.

It can only be diagnosed after death, which is what makes studying it so difficult. During life, symptoms can include memory loss, confusion, depression, aggression, impulse control problems, and eventually dementia. The study found that among players with the most advanced stage — Stage IV CTE — more than 90% also had dementia at the time of death.

Former NFL players die from neurodegenerative diseases at roughly four times the rate of the general population, according to the same research team's prior work. CTE affects less than 1% of the general non-football population.

The Critical Detail About When This Damage Happened

One finding in the study challenges a common assumption about when and where the damage occurs.

Most of the players in the study played the bulk of their careers before 2005, when CTE was first diagnosed in a former NFL player and the league began taking the issue seriously. The NFL has since banned helmet-to-helmet tackles, eliminated traditional kickoffs, and invested significantly in safer equipment and concussion protocols.

But Daneshvar made a point that matters far beyond rule changes at the professional level. "The bulk of the head impacts that these former NFL players experienced didn't occur at the NFL level. They occurred at the collegiate level, at the high school level, and in many cases the youth level. And all of those cumulative hits to the head added up to result in an increased risk."

In other words, by the time a player reaches the NFL, the damage from years of youth and high school football may already be accumulating. Fixing the NFL doesn't fix the problem if the harm is built over decades starting at age 10.

Researchers are urging the scientific community to examine athletes who played more recently — those who benefited from improved protocols — to see whether the rate changes. That data won't exist for decades.

What This Means

The NFL has 1,696 active players on its rosters at any given moment. A quarter of a million young Americans play high school football every year. The study's findings don't mean every one of them will develop CTE — but they confirm that the sport's relationship with degenerative brain disease is not a fringe concern or an overblown media narrative.

It's a documented, measurable public health reality that the scientific community is still working to fully quantify. The 25% floor from Monday's study is the most rigorously derived estimate the field has ever produced. Where the ceiling actually sits remains one of the most important unanswered questions in sports medicine.

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