In 2017, a bombshell discovery rocked the world of American football.
Alarmingly, scientists found that 99 percent of examined brains from deceased NFL players bore signs of chronic traumatic encephalopathy (CTE), a degenerative neurological condition linked with repeated blows to the head.
In the years since, studies have revealed numerous sports increase the risk of brain injuries from repetitive head trauma, including soccer and rugby.
In the case of CTE among former NFL players, though, it's been difficult to identify just how common the brain disease is, but a new study appears to give us the most accurate assessment yet, suggesting about three-quarters of professional players had the condition when they died.
"Current knowledge regarding CTE neuropathology in American football players is derived from autopsy-based studies," researchers report in a new paper published in JAMA Network Open.
"Autopsy studies are inherently susceptible to registry bias, which may distort estimates of both disease prevalence and associations of exposures with outcomes."
In other words, when scientists study the post-mortem brains of ex-NFL players that have been donated to science, their results are limited to those specific donations.
For example, the landmark 2017 study found signs of CTE in 110 out of 111 (99 percent) of brains studied – but that doesn't mean that 99 percent of all NFL players would develop the disease.
That's because players who experience cognitive, mood, or behavioral symptoms would be expected to be more likely to participate in brain donation, resulting in a form of selection bias (aka 'registry' bias in this context).
In August this year, researchers published a study that took some preliminary steps to account for that bias, identifying all 1,712 former NFL players who died during the years 2008–2021.
Of that group, 338 had donated their brains for study, and 315 of them had a diagnosis of CTE.
But because most of the former deceased players had not donated their brains for study, all researchers could conclude was that CTE prevalence at death ranged somewhere between 18.5 percent and 98.7 percent – or between 24.5 percent and 97.7 percent for players who died between 2016 and 2021.
In the new study, several members of the same research team refined these estimate ranges by combining data from the UNITE brain bank with population-level mortality records for all deceased NFL players identified through the National Death Index (NDI).
After modeling the selection bias effect – estimating that players with cognitive symptoms were approximately five times more likely to have their brains donated for study – the researchers calculated what they claim is the first estimate of population-level prevalence of CTE neuropathology among former NFL players at the time of their death.
It's a breakthrough. But for the NFL community, it's an undeniably grim statistic.
"After adjusting for this selection process, we estimated that approximately 74.5 percent of deceased former NFL players had CTE neuropathology at death between 2008 and 2023," the team explains in their study, led by biostatistician Julia Gallini from Boston University.
While the findings are certainly concerning, they don't mean that three-quarters of former NFL players are destined to develop dementia, says study co-author and neuroscientist Daniel Daneshvar from Harvard Medical School.
"In our clinic with former players, particularly those in midlife, we have had success improving cognitive symptoms, mental health and quality of life through both targeted treatments and proactive brain health strategies," Daneshvar explains.
"While we cannot yet treat the underlying CTE pathology, our goal is to bend the curve of brain aging, extend brain healthspan and help players feel and function better throughout their lives."
In terms of next steps, the researchers say it will be important to estimate the prevalence of CTE in football players at lower levels of play (high school and college), to help define the scope of this public health concern, but they note the lack of data may make that difficult.
Aside from the statistical challenges, they say we need more research into the mechanisms, clinical manifestations, and prevention of CTE across all levels of play.
The findings are reported in JAMA Network Open.
This article was fact-checked by Rebecca Dyer and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.