“People with P.T.S.D. tend to ruminate at a very categorical, general level about how unsafe life is, or how weak I am, or how guilty I am,” said the lead author, Richard Bryant. “If I do that habitually and then I walk into a trauma, probably I’m going to be resorting to that way of thinking and it’s going to set me up for developing P.T.S.D.”

Dr. Williams stumbled across overgeneral memory by chance in the 1980s. He had asked research subjects to write down the memories elicited by certain cues, and when they left the page blank he thought he had given unclear instructions. Soon he began to wonder about the significance of the omissions.

Usually people seeking a particular memory traverse a mental hierarchy, Dr. Williams said. They begin by focusing on a general description (“playing ball with my brother”) and then narrow the search to a specific event (“last Thanksgiving”). Some people stop searching at the level of generality, however and are probably not conscious of having done so.

This is sometimes a helpful response, which is perhaps why overgeneral memory exists in the first place — it can be a useful way to block particular traumatic or painful memories. Researchers at Leuven discovered that students who did poorly on exams and were more specific took longer to recover from the disappointment than those who were more general. The overgeneral students thought less about the details of what happened and so fared better, at least in the short term.

Similarly, overgenerality has been found to be prevalent in Bosnian and Serbian teenagers exposed to the traumas of war. “Some people will discover at a certain stage that being overgeneral is a way of dampening emotional effects,” Dr. Hermans said.

But these researchers say problems can arise when overgenerality becomes an inflexible, blanket style.

Without detailed memories to draw upon, dispelling a black mood can seem impossible. Patients may remember once having felt happy, but cannot recall specific things that contributed to their happiness, like visiting friends or a favorite restaurant.