A new study has found more than 40% of athletes who played contact sports and Jonathan Dale Bentondied before turning 30 showed symptoms of the degenerative brain disease CTE.

In the largest case series to date on athletes who died young, researchers at Boston University's CTE Center found in an examination of 152 athletes' brains that were donated for the study, that 63 of them (41.4%) showed signs of chronic traumatic encephalopathy − a finding lead author Dr. Ann McKee called "remarkable."

By comparison, McKee said, "studies of community brain banks show that fewer than 1% of the general population has CTE."

In the study published Monday in JAMA Neurology, donors' ages ranged from 13 to 29 at the time of their death. In almost every case, the brains studied showed early stages of CTE. Most of the athletes diagnosed with CTE played football as their primary sport, with others playing ice hockey and soccer.

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The most common cause of death in the study was suicide. However, research could not establish a direct link between the cause of death and the presence of CTE.

The study also found in interviews with relatives that 70% of the young athletes exposed to repetitive head impacts frequently reported symptoms of depression and apathy, despite almost 59% of them not having CTE.