Researchers from Boston University, Mass General Brigham and the Concussion & CTE Foundation reviewed federal death records for 2,362 former players who died in that period. The study estimated that 27.2% of them had dementia at some point in their lives. Among players who donated their brains to Boston University's brain bank, close to 400 in all, 93.5% showed CTE.

That donor figure alone would overstate the true rate, because players with cognitive symptoms were about five times more likely to donate. The team adjusted for that skew to reach the 74.5% estimate for all deceased players, including those who never donated. Boston University biostatistician Yorghos Tripodis, who led the study, said CTE appears in less than 1% of people not exposed to repetitive head impacts. "Now, we see among those who played (football) the longest, the prevalence of CTE is 75%," he said.

The estimate covers only deceased NFL players. It says nothing about living players or about those who played only youth, high school or college football, and the study did not examine symptoms tied to the disease. CTE can currently be diagnosed only by examining brain tissue after death, though researchers are working on a test for living people. The disease is classified in four stages, and symptoms often don't appear in the earlier ones. The researchers wrote that many cases "may have been early stage at the time of death and possibly symptom free." Julia Gallini, a doctoral candidate at the Boston University School of Public Health who worked on the study, called the 75% figure "worrying" and said it points to a need for better support, diagnosis and treatment for the NFL community.

Tripodis said the goal is to "make the game safer," and that "it's not that we are anti-football." He proposed reducing tackle football before high school and eliminating it from practice at all levels, noting that helmets and other equipment protect the skull but not the brain from rotational movement. The US Centers for Disease Control and Prevention says it remains unclear whether environmental or genetic factors raise the risk, or whether only severe concussions cause the disease or milder impacts do too.

The NFL acknowledged a link between football and CTE in 2016 and has since changed rules and equipment, including allowing padded Guardian Caps. In response to the study, the league reaffirmed its commitment to player safety and pointed to continuing efforts to reduce concussions and expand mental and physical health resources. The NFL Players Association said such research is critical, pledged continued support for players and families, and called for further investment in science and access to health care. It cited programs including the Football Players Health Study at Harvard University.

New England Patriots coach Mike Vrabel, a former NFL linebacker, acknowledged the risks and backed the league's safety efforts. Players "love the game so much that you know you're going to do whatever you can to play the game," he said.

CTE was once known as "punch drunk" for its association with boxing. Among the former NFL players found to have had it are Junior Seau, who died in 2012 at age 43, and Aaron Hernandez, who died in 2017 at 27 with Stage 3 CTE.