Key Takeaways
- Among the donated brains of deceased NFL players, 93.5% showed CTE — but donors are a self-selected group, so researchers modelled all 2,362 players who died between 2008 and 2023.
- After adjusting for the fact that players with cognitive symptoms were about five times more likely to have their brains donated, the estimated CTE prevalence was still 74.5%.
- Even a deliberately conservative version of the analysis put the lower bound at 63.7%; the figures cover all stages of CTE and apply only to deceased players.
The 93.5% CTE Number Had One Big Problem. Researchers Tried to Fix It.
Almost every donated NFL brain examined in major CTE studies shows signs of the disease. But there's an obvious objection: families may be more likely to donate the brains of players who showed memory problems, personality changes or other symptoms.
That could make the numbers look much worse than reality.
A Boston University research team has now attempted to measure exactly how much that bias matters.
The researchers started with all 2,362 former NFL players who died between 2008 and 2023, identified through National Death Index records — not just the 396 whose brains were donated.
Among the 387 donors with a determined CTE status, 93.5% had CTE pathology.
But the researchers also measured the donation bias. Using dementia-related causes of death recorded for donors and non-donors alike, they estimated that a player with clear cognitive symptoms was about five times more likely to have his brain donated.
After adjusting for that selection effect, the estimated CTE prevalence across all deceased players fell to 74.5% (95% CI, 69.6%–78.3%).
Even a conservative sensitivity analysis designed to produce a lower estimate landed at 63.7%.
There are important caveats. The estimate includes every stage of CTE, including early-stage cases that may never have caused symptoms. Symptoms among non-donors had to be inferred from death certificates, which undercount dementia. And the findings describe deceased players from 2008–2023 — they cannot be extended to living or current players.
For comparison, community brain banks find CTE in under 1% of general-population brains.
Health note: This article summarizes a research study and is not medical advice. Anyone worried about symptoms after head impacts — memory, mood or thinking changes — should raise them with a doctor rather than waiting.
The Twikup Take
The biggest argument against the brain-bank numbers was selection bias. Researchers measured it, adjusted for it and watched the estimated prevalence fall.
But it didn't fall nearly as far as skeptics might have expected.
Even under the conservative analysis, roughly two in three deceased former NFL players were estimated to have CTE pathology.
Image: AI-generated illustration.