Fear of devastating brain injuries has cast a shadow over America's most popular sport. Should children be allowed to play? Here's everything you need to know:

Why are some parents worried?
Recent research found that when children who play football and other contact sports suffer repeated jolts to the head, it can cause lasting damage to the developing brain. That can be true even when kids do not suffer any concussions. This was startling news, given that Pee Wee and Pop Warner players sustain from 240 to 585 head hits per season between ages 9 and 12, a critical period of brain development. As a result, some prominent voices have urged parents not to let their kids play the game, among them neuropathologist Bennet Omalu, whose discovery of the progressive brain disease chronic traumatic encephalopathy (CTE) in former NFL players is depicted in the film Concussion. Omalu contends that children under 18 should not be allowed to risk their future by playing football. "Our children are minors who have not reached the age of consent," he says. A growing number of athletes now agree, including the hard-nosed former NFL tight end and coach Mike Ditka, who says that if he had a young son, he wouldn't let him play football. "That's sad — and my whole life was football," Ditka says, but "I think the risk is worse than the reward."

What does the research show?
Some of the more troubling findings came from a 2015 study at Boston University. Researchers scanned the brains of 42 former NFL players, half of whom began playing tackle football before they turned 12, and the other half afterward. The ones who started younger performed worse on cognitive exams, while MRI testing revealed substantial abnormalities in their corpus callosum, which relays commands and information between the brain's two hemispheres. In another study, Wake Forest University researchers detected brain matter anomalies among 24 high school football players tracked over the course of a single season, during which none had suffered a concussion — a loss of consciousness or extreme confusion. "I'm more concerned about repetitive hits we refer to as nonconcussive trauma," says Robert Stern, author of the BU study. A player may display no outward abnormalities, but his brain "is jostled over and over again inside the skull," with the resulting neuron damage eventually causing CTE.

What are the symptoms of CTE?
They can include mental confusion, loss of intelligence, headaches, aggressive behavior, dementia, severe depression, and suicidal thoughts. CTE, however, can only be diagnosed during an autopsy, when doctors can search for telltale signs, notably excess deposits of a corrosive protein called tau. It was not until 2002 that Omalu confirmed the first case in a football player: Mike Webster, former center for the Pittsburgh Steelers who had died of a heart attack at 50 after suffering from depression and dementia. A Department of Veterans Affairs study has since analyzed the brains of 91 deceased NFL players and found 87 with CTE. Other prominent victims have included Chicago Bears safety Dave Duerson and Hall of Fame linebacker Junior Seau, both of whom committed suicide after suffering crippling depression; scores of other ex-players display symptoms. Fear of CTE prompted San Francisco 49ers linebacker Chris Borland, who had suffered a minor concussion, to retire last year after just one season.

What does the NFL say?
The league initially dismissed Omalu's findings, but finally acknowledged there was a problem — and settled a class action suit by former players that could ultimately cost $1 billion. Commissioner Roger Goodell claims a range of safeguards, such as reduced head contact in drills and improved equipment, have cut concussions "by 35 percent." But after promising the National Institutes of Health to fund a $16 million effort to diagnose CTE in living patients, ESPN reported, the NFL backed out because it was not happy the project will be led by BU's Stern, who has been critical of the league. Meanwhile, publicity over head trauma may have contributed to a 9.5 percent drop in Pop Warner participation between 2010 and 2012. The decline has since leveled off, however — good news for the NFL, since 70 percent of their players come up through the program.

Can better helmets protect kids?
Companies have developed helmets with more cushioning, but that's no panacea. No amount of protection can stop the sudden acceleration and deceleration of the head that comes with blocking and tackling, which can cause the brain to slosh around inside the skull like the yolk inside a vigorously shaken eggshell. When the brain is compressed against the skull by this sudden stopping and starting, neurons are damaged — and they never recover. In light of this reality, kinesiologist Erik Swartz of the University of New Hampshire last year persuaded the school's Division I team to allow half its players to drill without helmets. The idea was that they would instinctively modify their blocking and tackling technique because any head contact would really be painful. By season's end, the UNH players who had trained helmetless had experienced 30 percent fewer head blows. "It sounds simple," Swartz says, "but if there isn't contact to the head, there won't be a head injury."

The youngest CTE victim
In Harrisonville, Missouri, Michael Keck was a hometown hero. Blond and handsome, the bruising linebacker started playing football at 6 and seemed impervious to pain, believing that if you're hurt, "you just get out there — don't be a wuss," his wife, Cassandra, recalls. Keck's reckless tackling earned him a scholarship to the University of Missouri, but in his freshman year a crushing head blow knocked him unconscious. Tortured by head and neck pain, blurred vision, and constant ringing in his ears, Keck transferred to Missouri State, took one last massive hit to the side of his head, and then quit the game. Despondent and anxious, he experienced disorientation and memory loss. At age 25, Keck died of cardiac arrest, in 2013. Before his death, he had made Cassandra promise to donate his brain to the CTE lab at Boston University. Researchers found it brittle, deformed, and riddled with tau protein — a classic case. Keck is the youngest person definitively diagnosed with CTE.