Dr. Samuel Gandy, director of the N.F.L. neurology program at Mount Sinai Medical Center, said that rage and irritability “are far and away the most prominent symptoms” among former players with likely C.T.E., in his research. His group has identified 10 of 24 former players who probably have C.T.E.

Scientists at Boston University, who reported the findings on Mr. Hernandez, have described similar behavior in many of the more than 100 players they have evaluated. The caveat for both research efforts is that these samples are selective: Almost all of the players had signs of possible C.T.E. before being studied, which led the players and their families to participate and to donate their brains for research.