NICK SCHIFRIN: For years, researchers have studied the effects that repeated blows to the head have on athletes in pro football, hockey, soccer and other sports.

Now, as John Yang tells us, they're turning their attention to players who start as early as grade school.

JOHN YANG: Researchers at Boston University are conducting the first major study of CTE, chronic traumatic encephalopathy, in athletes who died before their 30th birthday.

CTE is the degenerative brain disease linked to repeated hits to the head.

Researchers recently released findings based on the study of the brains of 152 athletes; 63 of them, more than 40 percent, had CTE; 48 of them played football, most of them no higher than high school or college.

Some started playing tackle football when they were as young as 6.

The main sports for others in the study included hockey, soccer and wrestling.

Some of the parents of these young athletes told The New York Times about the changes they saw in their children.

MAN: His whole person started to change.

WOMAN: It was an abrupt change.

He just was -- he was a different person.

MAN: He wasn't the same kid.

MAN: He became unrecognizable.

MAN: Just a fragment of himself.

WOMAN: He wasn't Wyatt.

MAN: Then we couldn't figure it out.

MAN: And, unfortunately, we couldn't recognize possibly what was happening with him until it was too late.

JOHN YANG: That video is part of an interactive story on The New York Times Web site.

Times reporter John Branch worked on the project.

John, we heard those parents talk about these changes that they saw, this unrecognizable children.

Did any of them make the connection to football?

JOHN BRANCH, The New York Times: Yes, most of them did not, and that's not uncommon.

I think it's because they're so young that they weren't assuming that that was the cause of their problems.

These kids were 18, 19, maybe into their early 20s.

And so kids are going through a lot of changes at that time anyway.

So, they wondered if they were just typical changes that we see for if the kids were now using drugs or alcohol, maybe even just running in with the wrong crowd.

They couldn't make sense of it, unfortunately, really until it was too late.

JOHN YANG: And not only couldn't the parents not make sense of it.

Some of the athletes couldn't make sense of it.

JOHN BRANCH: Yes, that's right.

I mean, one of the clips I believe you played was the Bramwell family.

And Wyatt Bramwell did not tell anybody this until he recorded a video before his suicide, which -- in which he said that he had heard voices in his head, demons in his head, that his life had been a living hell because of what he assumed or the concussions that he had absorbed over several years of youth and high school football.

JOHN YANG: CTE, of course, can't be diagnosed until after death.

They can examine the brain after death.

These parents sent -- donated their children's brains for research because they were seeking answers.

What was their reaction when they got the confirmation that, yes, their children -- their child had CTE?

JOHN BRANCH: Yes, I think reactions are really mixed, of course.

I think there's a little bit of horror and regret.

They think, oh, my gosh, what have I done to my child?

What did I do to my child?

What could we have done to prevent this?

For the most part, they're the ones who (AUDIO GAP) their kids into football.

Some of them coached their kids in football.

And now to find out that they had died and that they had CTE tells them that what -- all the things that they thought that they were doing right for their kids may have actually been one of the causes of death for their kids.

Also, when they get the diagnosis, there is a strange bit of relief because of all the answers that they could not quite get or find, they were searching through doctors or therapists.

Now they know that there's a scientific explanation, at least in their minds, for what happened to their children.

JOHN YANG: You talk about how -- what they could have done differently.

You also ask the parents, if, knowing what they know now, they would have still had their child or let their child play football.

The first couple we're going to hear from are the parents of Hunter Foraker, who played at Dartmouth and died when he was 25.

MAN: If we have the knowledge today, Hunter would have never played football, period.

If I have a grandson, he will not play football.

It's not worth the risk.

WOMAN: When I drive by a football field and see these young kids in helmets and pads, it just -- it just breaks my heart.

WOMAN: Delaying the onset of contact is what I would have championed.

And I still love football.

We still have a son that plays football.

MAN: Does it hurt that I lost my son?

One hundred percent, it does.

But if you were to ask me today how I feel, like I said, I have grandsons now that love football and are playing contact football before high school, I would be willing to say that I would do it again.

JOHN YANG: That last couple, we should note, is the parents of Meiko Locksley.

And his father, Michael Locksley, is the head football coach at the University of Maryland.

Interesting division there, some saying, I would never have let my child play football, others saying, my grandchildren are playing tackle football.

What do you make of that?

JOHN BRANCH: Yes, Michael Locksley is a fascinating case, because he coaches football.

He coaches 100 young men to play this game, knowing that his son had CTE.

And to Michael and to many of these others, it's a strange calculation, an impossible calculation between risk and reward.

While many of us might say, oh, of course, we would never let our kids play football, knowing that they might end up with CTE, families, even these families, are a little bit more torn than that,because they wonder -- or they look at all the rewards that football can offer, the things like the camaraderie, the physical fitness, the sense of community, especially in small towns in America.

They don't want their kids to be ostracized in certain places for not playing football or not being part of it.

They love the game.

And so they're trying to balance those kinds of rewards for the risk.

And the tricky thing with CTE is, we don't know exactly what the risks are.

We don't know if a child who plays high school football has a one in a hundred chance of developing CTE, or is it one in 20, or is it one in 10?

To some, no matter what the risk is, that's too much.

But to others, they're trying to figure out where that lies and what's best for their own children.

JOHN YANG: Do you think this is going to lead to changes in youth football, or is the sort of the cultural pull of football in America just too great?

JOHN BRANCH: Yes, it's an interesting question.

I think (AUDIO GAP) having a conversation.

And I think there's a push more and more for having these conversations about whether children, especially before high school, should play tackle football.

The researchers behind this believe that there's a correlation between the number of years that somebody plays tackle football and the likelihood of them getting CTE.

And most kids never go on to college and certainly not out of the pros.

They would like to see that number limited by at least not having people play or kids play tackle football until, say, high school.

In the case of Wyatt Bramwell, who took his own life at age 18, he played 10 years of tackle football.

Would it have been different had he played three years, only in high school?

We don't know exactly, but that's the kind of limitations that I think a lot of people would like to see and we're going to be talking about in the next few years.

JOHN YANG: John Branch of The New York Times on an important subject to be talking about, thank you very much.

JOHN BRANCH: Thank you.