Currently, the best treatments for trauma require sharing the story of what happened. Talk is therapy—but when the things we share are horrifying, our listeners can be altered for the worse. In this way, individual trauma can morph into something collective.

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Michael, who used to be a jazz percussionist, has the slight hunch of a former drummer and the soft white hair of a television physician. He tends to bob his head when he’s thinking. No clients attended his first post-9/11 clinic, he told me, because he’d set it up on the New Jersey waterfront in a makeshift hospital on the very day of the attack. Unable to reach patients that day, he could only watch as the familiar Manhattan skyline disintegrated.

The lull would not last. In the days following the attacks, Michael began seeing eight to 10 patients a day for roughly hour-long sessions, five days a week. All had acute stress disorder, the label given to the extreme emotional shock that follows a tragic event. If acute stress disorder goes unabated, it becomes PTSD. Some of his patients had escaped the collapsing buildings, or were first responders who sorted through the rubble after they fell. Many were plagued by images they could not forget, and by survivor’s guilt.

Characterized by volatility of emotions, hyperarousal, pervasive fear, and anxiety, trauma is an adaptive response whose aim is simply to keep a person away from similarly dangerous situations in the future. But in the long run, it can leave the victim emotionally distraught, forever alert to new, illusory threats.

The PTSD that follows is a disorder of association. Sounds, smells, images, and thoughts of the traumatic event will elicit a fight-or-flight emotional response long after the trauma has passed, as with the war veteran who jumps when a car backfires.

The best treatments for PTSD target these associations. Traumatized patients are encouraged to confront their associations, often by purposefully reliving the traumatic event in order to experience their full emotional and physical reactions. As patients recount their stories multiple times a session, week after week, the associations of the event can lose their force. Ideally, their reactions will weaken with each telling.

PTSD therapy can be a transformative process for the speaker, but its effect on the listener can be more complicated. “Service providers often must share the emotional burden of the trauma,” writes Brian Bride, a professor of social work at Georgia State University; they “bear witness to damaging and cruel past events, and acknowledge the existence of terrible and traumatic events in the world.” Hearing stories of suffering, in other words, can generate more suffering.

During lectures at psychiatric grand-rounds in medical centers, Michael often asks his audience of physicians and medical students to imagine a lemon. “Hold it in your mind,” he says, “See how yellow it is. Smell the citrus aroma. Now cut a slice off with a knife and take a bite. Taste the strong sour flavor.” When he asks people to raise their hand if they are salivating, nearly all do so. The point of the experiment is simple: What you think and imagine can result in a demonstrable, physical reaction. When a therapist for a patient with PTSD hears a story of violence, empathetic imagining can inadvertently trigger a physiological reaction similar to what the victim may have experienced: a racing heart, shaking hands, nausea, and other elements of the fight-or-flight response.