Another common expression of PTSD in adults obviously won't apply to children.
"You're not going to have a 2-year-old talk about a foreshortened future," Zeanah said. "They're very much in the present moment."
Of course, babies and some 2-year-olds can't even talk yet at all. PTSD is typically diagnosable starting at age 1, after traumatic events including invasive hospital procedures, car accidents, dog attacks or witnessing violent events, such as the Newtown massacre, Zeanah said.
Treatment of PTSD in young children varies depending on the age, he said. Methods include a focus on the caregiver to better respond to a very small child's needs, and exposing a slightly older child to trauma reminders until the child's response to it lessens, he said.
For psychiatrists, having standardized diagnostic criteria for PTSD in pre-schoolers means that researchers are talking about the same manifestation of the disorder, whether in Hong Kong or Montana, he said.
It provides a kind of uniformity so researchers can can try to come up with more effective treatments, he said.
"We're so far ahead of even where we were 10 years ago," Cohen said. "Kids do get better, and they go on to lead healthy, happy lives. ... There's reasons for parents to have hope."