CHICAGO -- A new study suggests as many as 16 million people suffer from an actual disease that contributes to road rage.

A University of Chicago expert said most people think enraged drivers just need an "attitude adjustment." But experts now think there's a "biology and cognitive science" to some of the bad behavior behind the wheel.

A study released in the Archives of General Psychiatry says not everyone who leans on the horn or throws things around suffers from intermittent explosive disorder. But when it does occur, doctors say, the pattern of angry outbursts starts showing up around age 14.

According to the National Institutes of Health, which has funded research into the disorder, "people with IED may attack others and their possessions, causing bodily injury and property damage. Typically beginning in the early teens, the disorder often precedes -- and may predispose for -- later depression, anxiety and substance abuse disorders."

The NIH study said that to be diagnosed with IED, "an individual must have had three episodes of impulsive aggressiveness "grossly out of proportion to any precipitating psychosocial stressor," at any time in their life.

"The person must have 'all of a sudden lost control and broke or smashed something worth more than a few dollars ... hit or tried to hurt someone ... or threatened to hit or hurt someone.'"

Harvard Medical School professor Ronald Kessler said the findings are "news to a lot of people." He said even mental health specialists don't grasp that some 7 percent of the population may be affected.