Former football star Aaron Hernandez' brain was riddled with damage from a degenerative brain disease linked with head blows, but that doesn't necessarily explain the troubles that plagued his young life.
The diagnosis comes from a Boston University researcher who has studied hundreds of brains from football players, college athletes and even younger players, donated after their deaths. Dr. Ann McKee announced Thursday she found evidence of severe chronic traumatic encephalopathy, or CTE, in Hernandez' brain. Her autopsy also found signs of early brain shrinkage even though Hernandez was only 27 when he hung himself in prison in April.
His lawyer filed a lawsuit against the NFL and the New England Patriots on Thursday, claiming they failed to protect their players' safety.
What's known about CTE and how it affects the brain:
DOES CTE MAKE PEOPLE VIOLENT?
CTE can affect areas of the brain involved with regulating behavior and emotions. Aggression, depression, memory loss and dementia are among symptoms in former football players whose brains were donated to research, and some died by suicide. But substance abuse and other illnesses may be linked with those symptoms and there is no proof that CTE-related brain damage causes those behaviors.
Hernandez was serving a life sentence for a 2013 murder when he died, and had been acquitted in two other killings. An associate had earlier accused Hernandez of shooting him in the face after an argument at a strip club. Hernandez' background included other aggressive behavior and drug use.
WHAT CAUSES CTE?
Repeated head blows from contact sports and military combat are the most likely causes, scientists say. These hits can cause the brain to ricochet inside the skull, damaging nerve cells. Researchers are seeking to identify genes that may make certain people susceptible to damage from head blows, and they believe drugs and alcohol might also play a role.
CTE can only be diagnosed after death. A defining feature is abnormal deposits of tau, a protein that occurs naturally in the brain but is displaced in CTE. Tau build-up can damage or destroy brain cells.
HOW COMMON IS CTE?
In an account published in July, McKee reported finding CTE in the brains of all but one of 111 ex-NFL players studied. Most donated brains from former college players studied also showed signs, as did 20 percent of donated brains from high school players, most of whom died by suicide or drug overdoses.
But experts say most people recover from repeated head blows and the true frequency of CTE in football, other sports, the military and the general population is not known. It is not known how many head hits Hernandez experienced; he was a star in high school and college football and played most of three seasons with the New England Patriots. He was released in 2013 after his arrest.
WASN'T HERNANDEZ TOO YOUNG TO HAVE CTE?
Most brains donated for research are from older players but McKee has found CTE in an 18-year-old and her recent study found evidence of the disease in three high school players.