At least one in four N.F.L. players, and likely a much higher percentage, had the brain disease when they died. Will anything change?

The actual prevalence, researchers said, may be much higher.

New research finds that at least one in four N.F.L. players who died in a six-year period had C.T.E., a degenerative brain disease caused by repeated impacts to the head. Our reporter John Branch unpacks the data and its implications for the sport.