Yet drawing a direct line from those basic findings to what people do out in the world is dicey, given the ineffable interplay between circumstance, relationships and personality.
What scientists — from such diverse fields as psychiatry, neurology and substance use — can say is that the arrows seem to be pointing in the same direction. A number of brain states raise the risk of acting out violently, and the evidence so far, while incomplete, suggests that C.T.E. may be one of them.
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.
It may still be that most of the athletes in violent sports who develop the signature brain pathology, especially at modest levels, are no more irritable than anyone else. But an important hint to the contrary comes from a more mature corner of brain science: dementia research.
People with advanced dementia often begin to act in uncharacteristically aggressive ways, as many caregivers can attest. In a recent study of dementia patients, Swedish researchers found that 97 of 281 dementia patients had a history of aggression.
Those who acted out earliest in the progression of their disease had so-called frontotemporal degeneration — that is, damage concentrated in the frontal and temporal lobes of the brain. This is where C.T.E. shows up, too.
In frontotemporal degeneration, “a purported association has been made with criminal behavior,” said…