Observational study finds 25% of former NFL players exhibit brain changes related to CTE — Evidence Review

Of the recently deceased former NFL players whose brains were studied, at least 25% showed changes linked to chronic traumatic encephalopathy (CTE), according to a new observational analysis; this estimated prevalence is notably lower than some earlier reports. Most related research broadly supports a link between repetitive head impacts and CTE, though prevalence estimates and interpretations vary; you can read more about the new study as covered by the original study source.

• The 25% prevalence estimate from the new study is lower than some previous high-profile findings, such as a 2017 cross-sectional study that reported CTE pathology in 99% of examined former NFL players' brains, suggesting selection bias and variations in methodology may influence estimates 2.
• Multiple studies consistently find a strong association between participation in contact sports and increased risk of CTE or related neuropathological changes, particularly for professional athletes with longer careers and greater exposure to repetitive head impacts 1 2 3 5.
• There remains debate around the exact prevalence, causality, and risk factors for CTE, with some systematic reviews calling for more prospective, controlled research to clarify the relationship between repetitive head impacts, concussions, and neurodegenerative disease risk 7 9 13.

Study Overview and Key Findings

This new study provides a population-level estimate of CTE-related brain pathology among former NFL players based on donated brain tissue, a method that helps address prior selection bias concerns. The research comes amid heightened public awareness and concern about the long-term neurological effects of football and other contact sports, especially as evolving safety protocols and rule changes aim to reduce head injury risks. Unlike some previous studies focused only on clinically suspected or high-profile cases, this analysis attempts to extrapolate the prevalence of CTE in the broader population of deceased professional football players, providing updated context for ongoing debates about sports safety and athlete health.

Property	Value
Study Year	2026
Journal Name	The BMJ
Population	Former National Football League players
Sample Size	338 former NFL players
Methods	Observational Study
Outcome	Prevalence of chronic traumatic encephalopathy (CTE)
Results	At least 25% had brain changes related to CTE

Literature Review: Related Studies

We searched the Consensus paper database, which contains over 200 million research papers, to identify studies examining CTE, concussion, and long-term brain health in athletes. The following search queries were used:

1. CTE brain changes football players
2. concussion long-term effects safety
3. current CTE research findings controversies

Related Studies Table

Topic	Key Findings
How common is CTE among football players and what factors influence prevalence estimates?	- CTE is frequently found in former professional football players, with some studies reporting very high rates among selected brain donations (up to 99% in some NFL cases), but selection and sampling biases can lead to overestimates 2 3 5. - Newer studies and reviews suggest more moderate prevalence rates and highlight the need for better-controlled, population-based estimates to understand true risk 5 7 9 13.
What are the long-term cognitive and neuropsychiatric effects of repetitive head impacts?	- Multiple concussions and repetitive head impacts are associated with increased risk of cognitive decline, depression, and other neuropsychiatric outcomes in some athletes, especially professionals 1 7 10 14. - Evidence for increased risk in former amateur athletes is weaker, with most studies not showing significant associations 7 9.
What is the pathology and mechanism of CTE and how is it diagnosed?	- CTE is defined neuropathologically by distinctive patterns of tau protein accumulation, especially around blood vessels and in specific brain regions, and is different from Alzheimer’s and other tauopathies 1 5 11. - Diagnosis is currently only possible post-mortem; research is ongoing to refine diagnostic criteria and identify potential biomarkers for clinical use 5 11 12 14.
What are the main controversies and research gaps in the understanding of CTE?	- Debate continues regarding the exact causal relationship between repetitive head impacts and CTE, the influence of confounding factors, and the true prevalence among all athletes 5 9 12 13. - Most published studies are case series or autopsy studies with selection bias; prospective, longitudinal, and population-based research is needed for definitive answers 7 12 13.

---

How common is CTE among football players and what factors influence prevalence estimates?

The prevalence of CTE among former football players remains a subject of ongoing debate. While several studies have reported very high rates of CTE pathology among selected samples—particularly in brains donated by families concerned about symptoms—the new study’s estimate of at least 25% among deceased NFL players is lower than prior high-profile reports. This discrepancy highlights the importance of considering sampling and selection biases, as studies relying on brain donations may overestimate prevalence compared to population-based analyses 2 3 5. Systematic reviews stress the need for more representative studies to clarify the actual risk for all athletes 7 9 13.

• The 2017 JAMA study found CTE in 99% of examined NFL players’ brains, but acknowledged potential selection bias due to the nature of brain donations 2.
• Several neuropathological studies confirm a high burden of CTE among professional athletes, but prevalence estimates vary widely depending on methodology and sample source 1 2 3 5.
• Some systematic reviews and recent analyses suggest that previous estimates may be inflated and call for more robust, unbiased epidemiological data 7 9 13.
• The new study’s approach of estimating prevalence across all deceased NFL players provides a more conservative, possibly more accurate, perspective on risk.

What are the long-term cognitive and neuropsychiatric effects of repetitive head impacts?

Research consistently links repetitive head impacts and multiple concussions to increased risk of cognitive impairment, depression, and other neuropsychiatric outcomes in professional athletes. However, evidence for similar risks among amateur athletes is less clear, with many studies finding no significant associations in that population 7 9 10 14. The clinical manifestations of CTE are variable and may overlap with other neurodegenerative or psychiatric conditions, complicating diagnosis and risk estimation 1 7 10 14.

• Multiple concussions are associated with higher risk of cognitive decline and mental health problems in some former professional athletes, but not consistently in amateurs 7 9 10.
• Some studies report that CTE and related symptoms may be accompanied by other pathologies, such as Alzheimer’s disease or TDP-43 proteinopathy, further complicating the clinical picture 4 14.
• Consensus statements emphasize the need for further research on long-term outcomes and highlight the limitations of current evidence 6 7.
• The new study does not directly address clinical symptoms, but its findings inform ongoing discussions about long-term risks for professional football players.

What is the pathology and mechanism of CTE and how is it diagnosed?

CTE is characterized by distinctive neuropathological features, including abnormal tau protein accumulations in specific brain regions and around blood vessels. These features distinguish CTE from other neurodegenerative diseases such as Alzheimer’s 1 5 11. Diagnosis currently requires post-mortem brain examination, though recent advances in imaging and biomarker research aim to enable in-life diagnosis in the future 5 11 12 14. The underlying mechanisms include repetitive mechanical forces causing brain injury and tauopathy, but the precise pathways and risk modifiers are still under investigation 1 5 11.

• CTE pathology is distinguished by perivascular tau aggregates at the depths of cortical sulci, absent in other tauopathies 1 5 11.
• Recent studies have further refined diagnostic criteria and identified unique tau filament structures in CTE 5 11.
• Current diagnostic standards rely on post-mortem examination; clinical diagnosis remains a major research goal 5 11 12 14.
• The new study contributes to the understanding of CTE prevalence by using established neuropathological criteria.

What are the main controversies and research gaps in the understanding of CTE?

Despite growing evidence for an association between repetitive head impacts and CTE, significant controversies remain regarding causality, prevalence, and individual risk factors. Many published studies are limited by selection bias, lack of appropriate control groups, and confounding variables such as genetic predisposition, substance use, and coexisting medical conditions 5 9 12 13. Systematic reviews and consensus statements call for large-scale, prospective, and longitudinal studies to clarify these issues 7 12 13.

• Debate persists regarding the degree to which repetitive head impacts directly cause CTE, and how much is due to other factors 5 9 12 13.
• The lack of population-based, longitudinal studies hinders precise risk estimation and identification of vulnerable subgroups 7 12 13.
• Variations in clinical presentation and co-occurrence with other pathologies complicate efforts to define and diagnose CTE 4 12 13.
• The new study’s methodological approach seeks to address some of these gaps, but ongoing research is needed.

Future Research Questions

Despite progress in understanding CTE and its associations with contact sports, many important questions remain. Further research is needed to determine exact prevalence rates, identify risk factors, develop clinical diagnostic tools, and evaluate the effectiveness of prevention strategies. Addressing these questions is vital for informing athlete safety policies and medical care.

Research Question	Relevance
What is the true prevalence of CTE in all former NFL players?	Accurately estimating CTE prevalence requires large-scale, unbiased studies of all deceased players, not just those whose brains were donated due to suspected disease. This will help clarify actual risk 2 5 7 9.
Which factors increase an individual athlete’s risk for developing CTE?	Understanding the contribution of factors such as years of play, concussion history, genetics, and other exposures is crucial for targeted prevention and risk reduction strategies 1 5 7 10.
Can CTE be diagnosed in living individuals using biomarkers or imaging?	Currently, definitive CTE diagnosis is only possible post-mortem; developing reliable clinical diagnostic tools would enable earlier intervention and improved management 5 11 12 14.
What are the long-term cognitive and mental health outcomes for amateur football players?	Most evidence for CTE risk comes from professional athletes; clarifying risks in amateur populations will inform youth sports safety and public health recommendations 7 9 10.
Do recent rule changes and safety protocols reduce the risk of CTE in football?	As football organizations implement safety measures, it is essential to evaluate whether these interventions effectively lower head impact exposure and subsequent CTE risk 6 8.