“Having the disease can make it more likely for you to be depressed and even kill someone or yourself, but we’ll never know if it was the only or the main cause of this tragic outcome,” said Adam M. Finkel, a quantitative risk assessor at the University of Michigan School of Public Health. “But the inability to prove that the disease caused any particular outcome should not be used to cast doubt on the broader point, that exposure to repeated head hits is strongly associated with a disease that increases various bad outcomes.”
Even if Adams, whose six-year N.F.L. career ended after the 2015 season, is found to have had C.T.E., that may provide only one clue as to why he killed himself and six others. The disease has been linked to a host of symptoms, including aggressive, impulsive behavior and even suicidal thoughts. In many cases, families and friends of players found to have had C.T.E. say that the symptoms were uncharacteristic of the person they knew and that they became more pronounced over time.
In this case, Adams’s sister, Lauren Adams, told USA Today that her brother, who was 32, had recently become unusually aggressive.
“His mental health degraded fast and terribly bad,” she said. “There was unusual behavior.”
The disease has also been tied to memory lapses, loss of focus and problems following directions and handling everyday chores. But researchers have found only associations, not causal links, between the disease and the many apparent symptoms.
It remains difficult and perhaps impossible to determine a motive after a suicide because so many factors can play a role, including persistent mental distress and drug use. Adams does not appear to have left a note that tried to explain his motives, and such messages are often considered unreliable.