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A Harvard Medical School study of 2,000 former NFL players found that 34 percent of those surveyed believe they have chronic traumatic encephalopathy, a brain disorder caused by repeated head injuries.

The study also said approximately 25 percent of surveyed players who believed they had CTE experienced suicidal ideation.

Published in JAMA Neurology on Sept. 23, the study was a collaboration between scientists at HMS, Harvard School of Public Health, Massachusetts General Hospital, Brigham and Women’s Hospital, and the Spaulding Rehabilitation Hospital.

“We did this study because there’s a tremendous amount of controversy about how common CTE is,” said Aaron L. Baggish, an HMS associate professor of medicine at MGH and a professor of medicine at the University of Lausanne in Switzerland.

With current scientific techniques, CTE can only be formally diagnosed post-mortem through a physical examination of the brain during autopsy.

A main focus of the study was to understand what CTE “means to former professional football players,” Baggish said.

“Perceptions of either being healthy or unhealthy are probably as important as whether those perceptions are reality or not,” he said.

The study’s director of epidemiological research initiatives, Rachel Grashow, said players who believed they had CTE reported having medical conditions relating to mental health — including low testosterone, depression, and mood instability — at a much higher rate than those who did not.

“People really don’t know that there are a lot of common conditions, whether you played football or not, that cause cognitive problems,” she said. “I think the difference is that for players, they can point to this thing, right? This condition that they don’t know if they have.”

According to HMS Chief of Brain Injury Rehabilitation Daniel H. Daneshvar, a co-author on the study, future research on CTE should focus on finding a method of diagnosing patients during their lifetime.

“Once we are able to determine with a better degree of accuracy who has CTE and who doesn’t, then we can determine the extent to which these treatable factors are causing the specific problems athletes are reporting versus contributing to those problems,” he added.

While the possibility of having CTE is often met with hopelessness, athletes who may be suffering from the condition can improve their thinking, behavior, and mood with clinical treatment, Daneshvar said.

“It’s that clinical experience that we have that helped drive the rationale behind this study to identify just how frequent these treatable factors exist amongst NFL players,” he said.

Chris J. Nowinski ’00 — co-founder and CEO of the Concussion Legacy Foundation and a former member of the Harvard football team, who was not involved in the research — praised the study for its investigation of an important but understudied condition.

“CTE is a disease that was ignored for a long time, and so we don’t know enough about it,” he said. “All CTE research is very important to fill in our knowledge gaps that have helped us learn how to diagnose it in living people and learn how to treat it.”