Before they left for war in March 2006, more than 500 Minnesota National Guard troops volunteered for an experiment.

They agreed to answer a battery of questions about their mental health before setting foot in Iraq. Now they're back home, and the experiment is in full swing again.

Last week, a team of psychologists from the Minneapolis Veterans Medical Center began contacting the volunteers for their first postwar follow-up, part of a two-year plan to monitor them for signs of post-traumatic stress.

The project, Readiness and Resilience in National Guard Soldiers, is one of the first in the nation to study the emotional lives of National Guard troops before and after combat, lead researcher Melissa Polusny said.

The research was well underway before the suicide of Minnesota Marine Cpl. Jonathan Schulze in January focused sharp attention on how well the U.S. Department of Veterans Affairs provides mental health care for those fighting the war.

For Polusny, the mystery isn't why some soldiers become haunted or incapacitated by their memories of war, but rather why so many others are able to bounce back, without lasting hardship.

"While it's common to have some initial disruption in their life when they come home, most soldiers will be resilient -- they won't develop mental health problems," said Polusny, a psychologist with the Post-Traumatic Stress Recovery Program at Minneapolis' VA hospital.

She designed the study to examine the secrets of their success.

It comes at a time when there's been so much attention to post-traumatic stress, some veterans say, that many people think it's practically inevitable.

"It's kind of become like the catch-all phrase," said Dan Gazelka, 24, of Bemidji, Minn., a sergeant in the National Guard who returned from Iraq in July. "People automatically throw that out as soon as they hear that you went to Iraq. If something bothers you ... [they] automatically assume, 'Oh, you have PTSD.'"

It's estimated that 12 to 20 percent of Iraq war veterans are affected by post-traumatic stress, according to the VA's National Center for PTSD.

Three signs of PTSD

There are three telltale signs of the disorder: Flashbacks of the traumatic experience, feeling detached and shut-down emotionally, and being hyper-alert for danger.

But with returning veterans, it's really a matter of degree, experts say.

After combat, many report some temporary symptoms -- trouble sleeping, headaches, nightmares, edginess, or feeling sad, guilty or distracted, according to the PTSD center. It's a "normal reaction to combat experiences," says the center's 2006 Guide for Military Families, and most people "will recover naturally over time."

It only becomes a diagnosable mental illness, Polusny said, when the symptoms "get in the way of living your life," at times leading to tragic results: suicide, car accidents, alcohol and drug abuse, family violence and broken marriages.

Yet experts say that it's far from universal. "Most everybody who starts to study the issue is amazed at how resilient human beings can be in the face of trauma," said Dr. Irving Gottesman, a University of Minnesota psychologist who has studied PTSD.

Psychologists already know why some soldiers are more likely to get through combat emotionally intact. A big factor, Polusny said, is strong support from family and friends.