New research reveals the reason may be the number of head injuries players receive over the course of their careers. Many experts are worried about the impact on young players.
It's off-season right now, but whenever offensive guard Marcus Martin steps onto a football field, he's knows it's time for battle. In one game, Marcus got a pretty good blow to the head.
"I just felt tired exhausted and I could barely open my eyes," said Marcus.
Repeated concussions among adolescent athletes is the primary reason one well-respected neuropathologist said kids younger than 18 shouldn't play football.
For Marcus, who just got a scholarship to USC, that's unimaginable.
"Well, I think football you learn so much from football. You gain friends life lessons you know how to work hard take orders from somebody," said Marcus.
The concern: Chronic Traumatic Encephalopathy (CTE), the result of repeated head injuries over an extended period of time. It can cause mood disorders, paranoia and erratic behavior.
That may have been the case in the suicide of former Bears safety Dave Duerson. In final text messages to family, Duerson said he wanted to donate his brain to science and raise awareness about CTE.
"When it occurs in the adolescent brain it may actually change the trajectory of development," said Dr. Vernon Williams, medical director of the Kerlan-Jobe Center for Sports Neurology.
Williams says the research underscores the need for every sports program to provide proper equipment, enforce safety rules and educate each player about the symptoms of a concussion.
"If they develop difficulty with blurry vision or headache, nausea, difficulty with memory, difficulty with balance, any of those kinds of things after biomechanical force to the brain, meaning a hit, being thrown to the ground," said Williams.
Williams says with proper concussion management, he believes the benefits of organized sports outweigh the risks.
Marcus Martin plays hard, but he also plays smart. Head-butting is no longer allowed in the NFL, and that's perfectly fine with him.
"If you're getting paid that much money, I guess you are able to follow some of those rules," said Marcus.
Williams also adds that as hard as it is, it's very important for players to sit out games after a head injury.
Eleven states have passed legislation requiring a formal concussion management system in high schools, and California is one of them.