The connection between head injury and violence was particularly strong if the head injury had occurred within the past year, the authors of the study note in the journal Pediatrics.

According to the U.S. Centers for Disease Control and Prevention, some 1.7 million Americans experience a traumatic brain injury every year, due to bumps, blows, jolts, or any injury that disrupts the brain's normal functioning.

The study author, Dr. Sarah Stoddard with the University of Michigan in Ann Arbor, told Reuters Health that- with this type of research- it is difficult to figure out if brain injury is really the root of the aggression or if some other factor is the reason. Stoddard also notes that activities like drinking, drug use ,and a history of violence didn’t seem to explain the findings.

Stoddard and a colleague analyzed several years' worth of data from 850 kids in high school and followed them until five years after they left school. All of the participants had a grade point average of 3 or lower, putting them at risk for dropping out.

In the fifth year of the study, 88 of the young adults said they had suffered a head injury. Of those individuals, 43 percent said they had gotten into a fight, hurt someone, or taken part in some type of violence over the following year. That compared to 34 percent of those who didn't report a head injury. The findings suggest that the more recent a head injury is, the more likely a young adult is to be aggressive. According to Stoddard, "The brain does recover over time."

Stoddard also adds that researchers should investigate the long-term effects of head injuries in young people, as well as preventive measures such as protective gear for sports and interventions that help kids with head injuries manage their behaviors before they lead to violence.

A different study conducted by researchers at the Center for Injury Research and Policy Institute at Nationwide Children's Hospital, of young athletes 15-to-24 years old, reveals that sports are second only to motor vehicle crashes as the leading cause of injury to the brain. And concussions represent 10 percent of all high school athletic injuries.

Previous studies have also shown that brain injuries can also cause changes in memory, reasoning, and emotions, including impulsivity and aggression. In studies with prisoners, researchers have found that those with a history of brain injuries are more likely to engage in violence.

The study "does suggest there is a link between head injury and violence particularly early on," said Dr. Huw Williams, who has found the same relationship in prisoners, but was not involved in the new work. And if they believe their children experienced a brain injury in the past, they should also get expert advice on what to look for to make sure brain function doesn't deteriorate, he added. "It's important to monitor."

Brain injury can range from mild to severe causing a short loss of consciousness and confusion to amnesia and coma.

The American Academy of Pediatrics says that head injuries should be observed, and treatment should be sought if any of the following symptoms appear:

•       A constant headache, particularly one that gets worse

•       Slurred speech or confusion

•       Dizziness that does not go away or happens repeatedly

•       Extreme irritability or other abnormal behavior

•       Vomiting more than 2 or 3 times

•       Stumbling or difficulty walking

•       Oozing blood or watery fluid from the nose or ears

•       Difficulty waking up or excessive sleepiness

•       Unequal size of the pupils (the dark center part of the eyes)

•       Double vision or blurry vision

•       Unusual paleness that lasts for more than an hour

•       Convulsions (seizures)

•       Difficulty recognizing familiar people

•       Weakness of arms or legs

•       Persistent ringing in the ears

If your child does well through the observation period, there should be no long-lasting problems. Remember, most head injuries are mild. However, be sure to talk with your child's doctor about any concerns or questions you might have.

The Center for Disease Control and Prevention’s website, www.cdc.gov/traumaticbraininjury also contains a free online training course on preventing sports-related brain injuries in young athletes.