The Yale University researchers also found that combat vets were at much higher risk for divorce, depression and unemployment.

The findings are "striking," says study co-author Holly G. Prigerson, an associate professor of psychiatry at Yale. "Being exposed to and witnessing these horrible things puts you at a risk of a lot of bad outcomes for a long time."

The researchers examined a 1990-1992 study of psychiatric disorders of 2,583 men who were then between the ages of 18 and 54. About 7 percent -- 179 -- reported combat experience. The study was published in the Archives of General Psychiatry in 1994.

The combat vets were 4.4 times more likely to have abused a spouse or partner as other men, and were 6.4 times more likely to suffer from post-traumatic stress disorder, the researchers found. They were also two to three times more likely to suffer from depression, substance abuse, unemployment and divorce or separation.

Using a mathematical formula, the researchers determined that if there were no veterans in this country with combat experience, the number of domestic violence cases would drop by an estimated 21 percent.

"This really dramatically illustrates the price that we pay for sending people to war. It quantifies it," Prigerson says. "People know that war is bad, but do they know that 21 percent of spousal abuse could have been averted if men didn't go to war?"

One common assumption has been that post-traumatic stress disorder, not combat itself, is the prime cause of adjustment problems after wartime service. But the Yale study, which appears in the January issue of the American Journal of Public Health, shows that veterans still had problems, particularly with substance abuse and unemployment, even if they weren't diagnosed with the disorder.

The research reinforces what experts already know about the challenges facing combat vets, says Dr. Frank Schoenfeld, director of the Post-Traumatic Stress Disorder Clinic at the VA Medical Center in San Francisco.

Whether or not they develop post-traumatic stress disorder symptoms, combat vets frequently suffer from a lower tolerance for frustration, explosive outbursts and never-ending alertness, he says.

"There was some kind of intuitive understanding that people in combat tend to be irritable in general, and have fewer restraints on their anger," he says. "This helps verify that."

Both Prigerson and Schoenfeld say it's important to understand how combat affects a person's life in later years.

"If you send people off to war, it's going to have repercussions," Schoenfeld says. "Even if people don't come out diagnosed with a psychological disorder, they may come back with changes that are detrimental to society."

What to Do: Learn more about the diagnosis and treatment of post-traumatic stress disorder from the National Institute of Mental Health or the National Center for PTSD.