What the study measured

In a study of 176 professional boxers and MMA fighters who were retired, at least 35 years old, or both, 72 participants - 41 percent - met research criteria for TES, or traumatic encephalopathy syndrome. Boxers made up 62.5 percent of the full study group and 83 percent of those classified as TES-positive.

That distinction matters more than it might seem. TES is not a confirmed disease diagnosis. It's a symptom-based research classification - a checklist covering substantial exposure to repetitive head impacts, cognitive or behavioral impairment, and the absence of another condition that would better explain the symptoms, such as Alzheimer's disease. The criteria were proposed by a National Institute of Neurological Disorders and Stroke panel convened in 2019, specifically as an attempt to identify, in living people, who might have chronic traumatic encephalopathy.

CTE itself is defined by a specific pathological marker - an abnormal buildup of a protein called hyperphosphorylated tau, clustered in a particular pattern around blood vessels in the brain - and that marker can currently be confirmed only by examining brain tissue after death. Meeting the TES criteria while alive is not the same thing as having that underlying pathology, and the two shouldn't be used interchangeably, however closely related they're designed to be.

The fighters classified as TES-positive did share real, measurable differences from the rest of the cohort: more professional fights on record, more reported knockouts, lower volumes in several brain regions on MRI, and weaker performance on certain cognitive tests. Those are genuine findings. What they establish is a correlation between fight exposure and a research-defined clinical profile - not a confirmed prevalence rate for CTE itself.

What 41 percent does not mean

That distinction has become considerably more important in the past year. A 2026 study published in Nature Medicine by researchers at the University of Pennsylvania tested how well the TES criteria predict confirmed CTE pathology at autopsy. The researchers worked from a considerably larger dataset than the fighter study - 1,038 brain donors total, with detailed clinical records from life compared against pathology findings after death. Among that group, 25 would have met the TES criteria based on their clinical records from life. Of those 25, just six - 24 percent - showed the tau protein deposits that define CTE when their brains were examined after death. Separately, seven other individuals in the full dataset had CTE pathology even though they did not meet the TES criteria when researchers applied them retrospectively, meaning the checklist missed real cases in one direction while over-identifying in the other.

The Penn researchers weren't studying the same fighters, and their finding doesn't undo what the fighter study found about fight exposure correlating with cognitive and structural differences. But it does mean the 41 percent figure needs to be read as exactly what it is: the share of a specific group that met a research classification, not a confirmed CTE prevalence rate. As the study's lead author put it in the paper's release, someone who meets the current clinical criteria is more likely not to have CTE than to have it. In that autopsy sample, a positive TES classification corresponded to confirmed CTE pathology only 24 percent of the time - applying that label with more certainty than the science currently supports carries a real cost of its own, including to fighters who get told they likely have a progressive brain disease based on a checklist that performed this unevenly in the one large validation study run against it so far.

None of that erases the underlying concern. A knockout is a serious acute brain injury on its own terms, full stop - nothing in either study suggests otherwise, and nothing here should read as minimizing what a knockout does to the brain in the moment it happens. What the fighter study adds to that is a second, separate signal: TES classification tracked with age, professional fight count, and reported knockouts - pointing toward cumulative exposure as a meaningful factor, not just any single dramatic finish. The 25-plus-fight pattern the researchers flagged came with limited statistical sensitivity and specificity, which means it's a data point worth taking seriously, not a "safe" number of fights to stay under, and not a threshold anyone should treat as a guarantee in either direction.

Why suspension rules differ

Suspension rules vary by jurisdiction and by sport, and the coverage often blurs three different systems into one.

Federal law - the Professional Boxing Safety Act - applies specifically to professional boxing. It requires a boxing commission to recognize and enforce a medical suspension a fighter is already under from another commission; it doesn't create a single nationwide 30/60/90-day schedule that applies uniformly. The actual lengths, sparring restrictions, and clearance requirements come from state and commission-level rules, and those genuinely differ by jurisdiction and by combat sport.

New York's regulations are one detailed example: a fighter who loses by TKO with evidence of head trauma gets a minimum 30 days without sparring or competition, and can't return until neurological clearance has been documented and approved by the commission. The commission physician may issue longer suspensions at their discretion, depending on severity. Other states and other combat sports set their own specific terms. For professional boxing specifically, commissions report qualifying suspensions to boxer registries, and other commissions are required to recognize them - that's different from having one universal suspension schedule that covers every combat sport, and it's not a database a fighter "re-enters" so much as a record other commissions check before licensing a return.

What this means for fighters and the people around them

Cognitive or behavioral changes associated with repetitive head impacts can emerge during an active career or years after retirement - the fighter study included both active and retired participants, and it didn't establish one standard interval between exposure and symptoms. Delayed presentation is a recognized possibility in this research, not a guaranteed timeline, and the absence of symptoms during an active career is not itself evidence that cumulative exposure isn't accumulating in the background.

For an active fighter, the practical takeaway isn't a number of fights to avoid. It's treating suspension periods and sparring restrictions as functioning safeguards rather than an inconvenience to get past quickly, and tracking cumulative fight and sparring exposure deliberately rather than only watching the win-loss column. For anyone managing a fighter's career, the same logic applies to long-term planning rather than fight-by-fight decisions.

Anyone experiencing progressive changes in memory, judgment, mood, or behavior should seek evaluation from a qualified clinician regardless of fight history. A number of potentially treatable conditions can produce symptoms that look similar to what's described here, and - as the section above lays out - CTE itself cannot currently be confirmed while someone is alive, which is exactly why a proper clinical evaluation matters more than trying to self-diagnose off a checklist.

Where a brain injury results from legally actionable negligence and a valid claim exists, compensation may include ongoing medical costs, lost income and non-economic damages such as pain and suffering. Meeting TES research criteria, on its own, does not establish legal causation, liability or entitlement to compensation - those are separate questions a research classification doesn't answer.

Seeing the career beyond the fight

Millions' athlete directory gives fans a way to discover individual boxers and MMA athletes rather than encounter them only as a name on a fight card. That broader view matters here: a fighter's career includes training, recovery, and life outside competition - not only the finishes that make a highlight reel. Official fight records capture only part of that career, although the study itself did not quantify head impacts sustained during sparring.