BETHESDA, MD - A little-studied mental illness marked by episodes of angry, potentially violent outbursts like those in road rage or spousal abuse is more common than thought, researchers report to HealthDay News.  Known as intermittent explosive disorder (IED), it may affect up to 16 million people in their lifetimes.  In any year, IED may predispose people to other mental illnesses, such as depression and anxiety, and substance abuse problems.  The study, funded by the National Institute of Mental Health, appeared in the Archives of General Psychiatry.  "An awful lot of people in America have IED," said study lead author Ronald Kessler, a professor of health-care policy at Harvard Medical School.  "IED is characterized by explosive anger attacks they can't control and are out of proportion to what is going on in their lives and lead to physical assault or breaking things," he said.  According to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV), people with IED overreact to certain situations with uncontrollable rage, have a sense of relief during the outburst, and then feel remorse about their actions.  Kessler and colleagues based their findings on an analysis of data on 9,282 adults in the National Comorbidity Survey Replication done 2001-2003.  "We found IED is related strongly to depression and anxiety and other mental health problems," Kessler said 82% of those with IED were diagnosed with depression, anxiety, and alcohol or drug abuse disorders, although IED symptoms usually surfaced first.