"The idea that words (or smells or any sensory input) can trigger searing memories of past trauma—and intense fear that it may be repeated—has been around at least since World War I, when psychiatrists began treating soldiers for what is now called post-traumatic stress disorder. But explicit trigger warnings are believed to have originated much more recently, on message boards in the early days of the Internet. Trigger warnings became particularly prevalent in self-help and feminist forums, where they allowed readers who had suffered from traumatic events like sexual assault to avoid graphic content that might trigger flashbacks or panic attacks."