This Sept. 29, 2009 photo shows U.S. Marine Lance Cpl. Greg Rivers, 20, of Sylvester, Ga., holding his neck while waiting to take psychological tests at the Marine Corps Air Ground Combat Center in Twentynine Palms, Calif. The U.S. government is testing hundreds of Marines and soldiers before they ship out, in search of clues that might help predict who is most susceptible to post-traumatic stress disorder. (AP Photo/Jae C. Hong)