If the sport of football ever dies, it will die from the outside in. It won't be undone by a labor lockout or a broken business model — football owners know how to make money. Instead, the death will start with those furthest from the paychecks, the unpaid high school athletes playing on Friday nights. It will begin with nervous parents reading about brain trauma, with doctors warning about the physics of soft tissue smashing into hard bone, with coaches forced to bench stars for an entire season because of a single concussion. The stadiums will still be full on Sunday, the professionals will still play, the profits will continue. But the sport will be sick.

The sickness will be rooted in football's tragic flaw, which is that it inflicts concussions on its players with devastating frequency. Although estimates vary, several studies suggest that up to 15 percent of football players suffer a mild traumatic brain injury during the season. (The odds are significantly worse for student athletes — the Centers for Disease Control and Prevention estimates that nearly 2 million brain injuries are suffered by teenage players every year.) In fact, the chances of getting a concussion while playing high school football are approximately three times higher than the second most dangerous sport, which is girls' soccer. While such head injuries have long been ignored — until recently, players were resuscitated with smelling salts so they could re-enter the game — it's now clear that these blows have lasting consequences.

The consequences appear to be particularly severe for the adolescent brain. According to a study published last year in Neurosurgery, high school football players who suffered two or more concussions reported mental problems at much higher rates, including headaches, dizziness, and sleeping issues. The scientists describe these symptoms as "neural precursors," warning signs that something in the head has gone seriously wrong.

This research builds on...