For years he was tortured by a horrifying image of 9/11: elevator doors at the World Trade Center slide open, and burning people stumble out; screams fill the area. Except, he was not at the World Trade Center that day. A clinical psychologist, he had treated several patients who were there and suffered post-traumatic stress disorder (PTSD) as a result, unable to rid themselves of the terrifying memories. Over the course of long, tortured conversations, these memories etched themselves indelibly into his own mind. They intruded on everyday situations and turned up in nightmares. For the first time in his life he had panic attacks.

And he is by no means alone. In the past several years it has become evident that therapists, emergency personnel, the police and family members who deal with traumatized individuals can develop symptoms of PTSD secondhand. They endure what are called intrusions—images, flashbacks and nightmares that cause them to experience the horrible events over and over—even though the memories are not their own. Like people who have themselves been terrorized, they live in a state of stress-induced hyperarousal, with an overly active fight-or-flight response. They may suffer from sleep disorders and feel utterly hopeless.

The most recent edition of the Diagnostic and Statistical Manual of Mental Disorders acknowledges the problem. A diagnosis of PTSD no longer requires the immediate experience of a traumatic event; a person need not have been a victim or even an eyewitness. It is enough simply to hear the details. Recent research has begun to clarify how common the problem is and why some people are more susceptible to it than others.

Your Stress Is My Stress

The collected research suggests that 10 to 20 percent of people closely involved with those who have PTSD “catch” the condition themselves—with the numbers varying depending on the study and the group being investigated (such as therapists, social workers or family members). In 2013, for instance, a team led by Roman Cieslak of the Trauma, Health, and Hazards Center at the University of Colorado Colorado Springs Medical Campus found that almost one in five of more than 200 health care providers helping military personnel with PTSD met the criteria for “secondary trauma,” one name that researchers apply to the phenomenon.

A follow-up analysis concluded that the providers had about as many symptoms, such as intrusions, as rescue personnel or social workers who had been at the scene at the time. And according to psychologist Tamara Thomsen of the University of Hildesheim in Germany and her colleagues, one in five of approximately 300 trauma therapists who responded to an online questionnaire could be diagnosed with moderate secondary trauma—and one in 10 with severe secondary trauma.

In several studies involving family members, Israeli trauma researcher Zahava Solomon of Tel Aviv University found that a percentage of the wives of former prisoners of war could be diagnosed with indirect trauma. A 2017 review that included parents and children of war veterans, as well as committed partners, paints a more inconsistent picture, though: the partners were affected most frequently; parents seemed not to have been “infected,” and children sometimes exhibited symptoms, although they were not especially severe.

How is it that PTSD can be transmitted to caregivers or family members? At first glance it would seem quite remarkable that the sensory experiences of one person can end up in another person’s head. “In contrast to the victims of primary trauma, there is no direct input from the sensory organs that might be saved in memory in the brain,” observes psychologist Judith Daniels of the University of Groningen in the Netherlands. “There are only images.” But she has a possible explanation: “The regions of the brain that processes visual imagery have a very strong overlap with regions that process imagined visual experience.” In other words, at the processing level it may make little difference to the brain whether the images were created by the eyes and optic nerve or by the powers of imagination. “If this is how the processing works, then both may lead to visual intrusions,” she says.

Who Is Most Susceptible?

Another puzzle is why many therapists, caregivers and family members do not succumb to secondhand PTSD, whereas others do. Work by Thomsen’s group suggests that a strong capacity for empathy—the ability to identify with the feelings of others—may increase the risk of secondary trauma. In following up with their questionnaire respondents a year and a half later, Thomsen notes, the researchers found that therapists “who exhibited greater emotional empathy were more apt to experience secondary trauma at the time of follow-up.”

For family members of trauma victims, a lack of emotional distance may also contribute, as is suggested by the finding that wives of former prisoners of war are more vulnerable to indirect trauma if they identify with their husband and internalize his traumatic experiences.

Researchers are also pondering the possible role of earlier trauma in susceptibility to secondary PTSD, theorizing that the symptoms may represent the reawakening of a prior, primary trauma. Some even doubt that symptoms occur in the absence of earlier primary trauma. In this reawakening scheme, trauma can add up over a lifetime, with each additional episode increasing the risk of PTSD. Hearing about the traumatic experiences of another person may become the straw that breaks the camel’s back.

To Thomsen, this notion implies that it may be important to figure out whether symptoms in a given therapist reflect secondary trauma or retraumatization. Daniels, however, finds it implausible that personally experienced trauma could by itself account for indirect PTSD. As evidence, she points to a meta-analysis by Jennifer Hensel, then at the University of Toronto, and her colleagues. The analysis found only a slight relation between personally experienced trauma and development of secondary trauma, which implies that past history probably explains only a small portion of the intensity of someone’s symptoms. “So it’s not nothing, but it is far from an adequate explanation for how these symptoms arise,” Daniels says.