Any American motorist has probably heard things like: "You stupid son of a [blank] what the hell is wrong with you?"
When traffic is at a crawl, anger can go from zero to 90 miles per hour in no time.
"I get very loud, I get very belligerent, I start shouting," said road rage sufferer Nikki Cooper. "Everybody in this city probably knows the sound of my horn."
Doctors think Cooper's road rage was the symptom of something far deeper. She was diagnosed with a condition known as intermittent explosive disorder, or IED.
"Road rage, especially if it's frequent enough, is probably a part of IED, which is much more prevalent than people thought," said Dr. Emil Coccaro of the University of Chicago.
A new study of more than 9,000 people found that four percent had IED.
Harvard Medical School epidemiologist Ronald Kessler said he was "blown away by how many people meet criteria" for IED. He described it as "mind-boggling" that its prevalence hadn't been recognized sooner.
Intermittent explosive disorder, an imbalance in brain chemicals, affects up to one in 20 people -- more men than women. It is far worse than just having a bad temper.
"They're having low-level outbursts -- screaming, shouting, slamming doors -- or they're having at least three episodes a year where they're destroying large amounts of property, physically assaulting, hurting people," said Coccaro.
For those who suffer from IED, the most minor traffic trouble can drive them into rage that is difficult to control.
"It just feels like an explosion of rage," said Mary McPhee, who is diagnosed with IED. "And I try to stop it, and it doesn't stop
The study found few people suffer from IED alone. More than 80 percent of those with IED also have some type of mood, anxiety, impulse control or substance use disorder.
The type of road rage linked to IED can be controlled with medication and therapy.
For Cooper, it was a matter of learning what triggered her anger, and then learning how to hit the brakes.
"I don't, too much, retaliate anymore," she said.
The first anger attack is usually seen around 14 years of age. Kessler said anger is usually the first problem -- then people can become anxious, depressed, hostile or alcoholic later in life. While 60 percent of people with IED seek professional treatment for a mood or substance problem, only 29 percent receive treatment for their anger.
"Anger seems to be the core," said Kessler. "But it is not being treated, not being recognized."
Most people are surprised when they hear about IED. "People go, 'There's a name for it? There's something you can do? And it works?'" said Kessler. "You don't have to blame yourself for it -- it's a biological thing."
Kessler added, once you do know about IED, "you do have to be responsible for not doing anything [about it]."