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Stephen Casper, a medical historian, argues that the danger of C.T.E. used to be widely acknowledged. How did we unlearn what we once knew?

In July, 2015, Stephen Casper, a medical historian, received a surprising e-mail from a team of lawyers. They were representing a group of retired hockey players who were suing the National Hockey League; their suit argued that the N.H.L. had failed to warn them about how routine head punches and jolts in hockey could put them at risk for degenerative brain damage. The lawyers, unusually, wanted to hire a historian. A form of dementia called chronic traumatic encephalopathy, or C.T.E., had recently been posthumously identified in dozens of former pro football and hockey players; diagnosable only through a brain autopsy, it was thought to be caused by concussions—injuries in which the brain is twisted or bumped against the inside of the skull—and by recurring subconcussive blows to the head. In the media, C.T.E. was being described as a shocking syndrome that had never been noticed in sports outside of boxing. In essence, the legal team wanted a historian to tell them what science had known about head trauma, and when.

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