KELLY MCEVERS, HOST:
A degenerative brain disease called CTE keeps showing up in football players who have died. Just yesterday scientists revealed that Aaron Hernandez, who once played for the New England Patriots, had a severe form of the disease. Hernandez was found guilty of murder in 2015 and committed suicide in prison in April of this year. NPR science correspondent Jon Hamilton is with us now to talk about the case. Hey, Jon.
JON HAMILTON, BYLINE: Hey.
MCEVERS: So Aaron Hernandez was only 27 when he died. And he stopped playing football when he was 23, yet he was found to have severe CTE. Is that unusual?
HAMILTON: It is in the sense that most cases of CTE have been found in older, retired players. But that may be actually a little misleading because you can only diagnose this problem in someone who has died, and people don't usually die in their 20s. Also, there have, you know, really been other young players with CTE.
Some people might remember Chris Henry who played for the Cincinnati Bengals. He was 26 when he died. He had the disease. There was also a guy named Owen Thomas who played for the University of Pennsylvania. He was 21. And the scientists at Boston University even found CTE in one high school football player.
MCEVERS: OK. So it sounds like there is a clear system to diagnose this disease after someone has died. What else do researchers know about CTE at this point?
HAMILTON: Well, they know the full name. It's chronic traumatic encephalopathy. They know it's a degenerative brain disease. And they know it's a disease that seems to affect people who take a lot of blows to the head. What seems to happen is that an abnormal protein called tau starts to appear. Brain cells begin to die. Eventually the brain shrinks. And when that happens, people can start to develop these symptoms, tremors or memory problems or even emotional problems.
MCEVERS: So going back to this case of Aaron Hernandez - and I think people have the question that could CTE have caused him to behave violently?
HAMILTON: It's possible. And certainly an attorney for the Hernandez family is arguing that it did. But I think most scientists would be pretty cautious about making that link. You know, what they have observed is that people with CTE seem prone to being aggressive or explosive or impulsive. And they also know that a number of athletes with CTE have committed suicide. But you know, CTE doesn't necessarily cause those behaviors.
And of course Aaron Hernandez had a history of violence that went back many years before his death, presumably before he had severe damage to his brain. There's also one other thing that's just a little bit odd. A jury found that Hernandez was guilty of a premeditated, execution-style killing. And that's kind of the opposite of an impulsive act.
MCEVERS: What are brain scientists saying about what to do now when it comes to CTE?
HAMILTON: Well, one thing they're saying is that there's enough evidence now to think that this is a real problem. And I think they're gratified that the NFL has now acknowledged that there seems to be some kind of link. But they have a lot of questions. One of the things they don't know is how exactly blows to the head result in this damage to brain cells years later. Another thing they don't know is, what happens when you watch a group of football players over many years, or what happens when you compare the brains of football players to the brains of other people?
And this is all kind of difficult to study because NFL players are kind of a different group. They're physically and mentally different from other people. Aggression is part of this game. So is the use of performance-enhancing substances, and those can also affect a person's behavior. And right now researchers don't even know how to predict who is vulnerable to this problem. It's pretty clear there are some people who can get hit on the head a lot and really suffer no consequences. Other people are really vulnerable. And right now nobody knows how to tell which athletes are going to see their brains atrophy and which ones are going to be fine.
MCEVERS: NPR's science correspondent Jon Hamilton, thank you very much.
HAMILTON: You're welcome. Transcript provided by NPR, Copyright NPR.