JUDY WOODRUFF: Now: A new study raises even stronger concerns about the risks of brain injury from the country's most popular professional sport.

It's the focus of our weekly segment the Leading Edge.

And Hari Sreenivasan has the story from our New York studios.

HARI SREENIVASAN: We have known about the dangers of hard hits and concussions from football, and that many players are suffering from CTE, or chronic traumatic encephalopathy.

But, yesterday, the largest study to date concluded that 110 of 111 former NFL players who had died and donated their brains had CTE. These included former greats such as quarterback Kenny Stabler and linebacker Dave Duerson.

Researchers also looked at the brains of 202 former players, including high school and college athletes; 87 percent were found to have CTE.

The NFL released a statement.

It said in part: "We appreciate the work for the value it adds in the ongoing quest for a better understanding of CTE. The NFL will continue to work with a wide range of experts to improve the health of current and former NFL athletes."

Michael Alosco is a co-author of the study from the Boston University CTE Center.

Michael, thanks for joining us.

You and your researchers point out that this is not a random sample, that the group of brains that you were studying either came directly from the athletes or from members of their family who thought there might be something wrong.

That said, what does this information tell you? What does this data point to?

MICHAEL ALOSCO, Boston University CTE Center: This is the largest case series study to date of CTE.

You know, it's among football players. The sheer numbers, the sheer size of the individuals who had CTE is really concerning. It's really suggesting a link between prior participation in football and CTE.

It's really going to serve as a foundation, as a rich data source going forward to really study this disease and figuring out what causes it and identifying the risk factors and to really better understand it.

HARI SREENIVASAN: Help us understand what CTE does to the brain.

MICHAEL ALOSCO: It's a neurodegenerative disease, so a neurodegenerative disease similar to Alzheimer's, but it's very distinct from Alzheimer's disease.

What we see in CTE is this protein called tau. Tau is a protein that helps stabilize the cell, but in CTE, it becomes abnormal. And, specifically, it starts to collect around the blood vessels.

And it's located at the base of the (INAUDIBLE) kind of right below the bottom of the tissue of the brain. And it's a really unique presentation.

HARI SREENIVASAN: So, Michael, I know you sent us a slide. We're going to put that on the screen.

On the left-hand side, you kind of see a cross-section of a brain that's almost off-white. And then, on the right-hand side, you see a lot more dark edges. And that, I suppose, is where the CTE is.

But what — how does this affect a player's behavior?

MICHAEL ALOSCO: We see a lot of different types of symptoms in CTE. A lot of them are nonspecific, meaning that, you know, we see these symptoms across various conditions.

However, in CTE, we're commonly seeing behavior changes, so we're seeing aggressive behaviors, violent behaviors. We're seeing changes in mood, like depression. And we're also seeing changes in thinking and memory.

What we need to identify is what symptoms are specific to CTE. And by doing that, it will help us diagnose CTE in life and really characterize it.

HARI SREENIVASAN: One of the things I found interesting was that you actually found this in high school players.

And so that means that this wasn't just the big-time NFL superstars that were hitting each other with tremendous force, but does this have a cumulative effect?

MICHAEL ALOSCO: That was actually really surprising to us as well is, we're finding CTE kind of across the levels of play going from high school up to the people who play professionally, as you mentioned.

But we don't know how much is too much yet. You know, we don't know what it means if you just play high school football. We don't know what that means in terms of risk for CTE. We did find it in three individuals.

But we need to, you know, identify those people and follow them over time to see what happens to them.

HARI SREENIVASAN: Speaking of that, while this is obviously the gold standard, you have someone's brain to work with, but that's, unfortunately, posthumously.

Is there anything that science is doing now to be able to detect and diagnose and intervene while a player is still alive?

MICHAEL ALOSCO: Right now, we can only diagnose CTE after death, as you mentioned.

Our work at Boston University is also, though, studying living subjects, and we're trying to identify tools that look at the brain, that look at different proteins in the blood, trying to identify these tools that can detect CTE in life.

And we're really hopeful that, you know, in the next five or seven years, we will make a lot of progress on that front.

HARI SREENIVASAN: It's important to point out you're not just focused on NFL players. You have actually the brains of athletes from other sports as well?

MICHAEL ALOSCO: Right.

So, the paper that we're talking about is all about football players. But, really, it's a brain bank for all individuals who had some type of exposure to repetitive head impacts.

HARI SREENIVASAN: I think one of the questions that parents want to know is, should I pull my child out of football?

MICHAEL ALOSCO: So, that's the million-dollar question we always get.

And it's a very tough question, and the reason being is, we know there's a lot of benefits associated with playing sports and participating in sports. But this study and our findings, they raise a lot of concern.

But we need to do more future research. We need to know more about the disease, more about the risk factors for the disease before we can really make any informed recommendations about policy or about safety. We need to learn more.

HARI SREENIVASAN: All right, Michael Alosco, thanks so much for joining us today.

MICHAEL ALOSCO: Thank you for having me.