"The ability to increase the firing rate, though, comes with a hidden cost. Severe psychological trauma becomes a distinct possibility when military training overrides safeguards against killing: In a war when 95 percent of soldiers fired their weapons at the enemy, it should come as no surprise that between 18 and 54 percent of the 2.8 million military personnel who served in Vietnam suffer from post-traumatic stress disorder—far higher than in previous wars."