A person’s head collides with an object. Unprepared for the impact, the head jerks in a violent whiplash motion. The person collapses, rolling on the ground and holding his head, before rising slowly and unsteadily. Eyewitnesses testify that the person was confused or disoriented.

“By definition, that’s a concussion,” says Dr. Daniel P. Perl, a professor of pathology at the Uniformed Services University of the Health Sciences in Bethesda, Md., the American military’s medical school. ...

A spate of suicides and diagnosed cases of dementia involving retired N.F.L. players has prompted research to determine whether there is a correlation between constant blows to the head, which are endemic to football, and a degenerative brain disease known as chronic traumatic encephalopathy.

Testing of brain samples drawn from deceased former players by the Boston University Center for the Study of Traumatic Encephalopathy has found that 33 from the N.F.L., along with 17 who competed at other ...