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 ...