TWENTYNINE PALMS, Calif. – Two days before shipping off to war, Marine Pfc. Jesse Sheets sat inside a trailer in the Mojave Desert, his gaze fixed on a computer that flashed contrasting images.

Smiling kids embracing a soldier. A dog sniffing blood oozing from a corpse. Movie star Cameron Diaz posing sideways in a midriff top. Troops cowering for safety during an ambush.

A doctor tracked his stress levels and counted the number of times he blinked. Electrode wires dangled from his left eye and right pinky finger.

Sheets is part of a military experiment to try to predict who’s most at risk for post-traumatic stress disorder. Understanding underlying triggers might help reduce the burden of those who return psychologically wounded – if they can get early help.

PTSD is a crippling condition that can emerge after a terrifying event – car accident, sexual assault, terrorist attack or combat. It’s thought to affect as many as one in five veterans returning from Afghanistan and Iraq.

Military doctors have been mystified as to why certain troops exposed to bombings and bloodshed develop paralyzing stress while others who witness the same trauma shake it off.

Studies point to some clues: Childhood abuse, history of mental illness and severity of trauma. A social net and a coping strategy appear to offer some protection.

However, none of the factors explored so far are reliable predictors.

“Right now, we can’t determine with certainty who will and who won’t develop PTSD,” said Paula Schnurr, deputy executive director of the Department of Veterans Affairs’ National Center for Posttraumatic Stress Disorder.

“Perhaps with better measures, we can get closer.”