"We were very surprised at the degree to which repeated media exposure was so strongly associated with acute stress symptoms," lead author E. Alison Holman, associate professor of nursing science at the University of California, Irvine, said in a statement.
"We suspect that there's something about repeated exposure to violent images or sounds that keeps traumatic events alive and can prolong the stress response in vulnerable people. There is mounting evidence that live and video images of traumatic events can trigger flashbacks and encourage fear conditioning."
The study challenges assumptions about how people react to collective traumas, such as the idea that individuals must be directly exposed to an event to be at risk for stress-related disorders.
It also raises questions about the latest edition of the Diagnostic & Statistical Manual of Mental Disorders, which specifically excludes media-based exposure as a potential trigger for trauma response among non-professionals.
"In our prior work, we found that early and repeated exposure to violent images from the terrorist attacks of Sept. 11 and the Iraq War might have led to an increase in physical and psychological ailments up to three years [later]," said study co-author Roxane Cohen Silver, a professor at UC-Irvine.
"Our new findings contribute to the growing body of research suggesting that there is no psychological benefit to repeated exposure to graphic images of horror."
Researchers surveyed a national sample of 4,675 U.S. adults two to four weeks after the Boston Marathon to assess acute stress responses to the bombings, the degree of direct exposure to the bombings, indirect exposure through media and prior exposure to other recent community-based traumas.
The study, published in the Proceedings of the National Academy of Sciences, found people exposed to 6 or more hours per day of bombing-related media coverage in the week after the attack were nine times more likely to report high acute stress than those with minimal media exposure -- less than 1 hour daily.
Symptoms of acute stress include intrusive thoughts, feeling on edge or hyper-vigilant, avoiding reminders of the event and feeling detached from it, the study said.