STRESS is an inevitable part of our life. Yet whether our daily hassles include the incessant gripes of a nasty boss or another hectoring letter from the Internal Revenue Service, we usually find some way of contending with them. In rare instances, though, terrifying events can overwhelm our coping capacities, leaving us psychologically paralyzed. In such cases, we may be at risk for post-traumatic stress disorder (PTSD).

PTSD is an anxiety disorder marked by flashbacks, nightmares and other symptoms that impair everyday functioning. The disorder is widespread. At least in the U.S., it is thought to affect about 8 percent of individuals at some point during their lifetime.

Although PTSD is one of the best known of all psychological disorders, it is also one of the most controversial. The intense psychological pain, even agony, experienced by sufferers is undeniably real. Yet the conditions under which PTSD occursin particular, the centrality of trauma as a triggerhave come increasingly into question. Mental health professionals have traditionally considered PTSD a typical, at times even ubiquitous, response to trauma. They have also regarded the disorder as distinct from other forms of anxiety spawned by lifes slings and arrows. Still, recent data fuel doubts about both assumptions.

Shell Shock
PTSD did not formally enter psychiatrys diagnostic bible, the Diagnostic and Statistical Manual of Mental Disorders (DSM), until 1980. Yet accounts of syndromes that mirror PTSD date back to Sumeria and ancient Greece, including a mention in Homers Iliad. In the American Civil War, veterans suffered from soldiers heart; in World War I, it was called shell shock, and in World War II, the term used was combat fatigue. In the 1970s some soldiers returning from the war in Southeast Asia received informal diagnoses of post-Vietnam syndrome, which also bore a striking resemblance to the DSMs description of PTSD.

According to the DSM, PTSD occurs in the wake of traumadefined by the manual as an extremely frightening event in which a person experiences or witnesses actual or threatened death or serious injury, or a threat to the physical integrity of self or others. (Less violent experiences such as serious relationship or financial problems do not count.) The most frequent triggers of PTSD thus include wartime combat, rape, murder, car accidents, fires, and natural disasters such as tornadoes, floods and earthquakes.

PTSD is now officially characterized by three sets of symptoms. These include reliving the event through intrusive memories and dreams; emotional avoidance such as steering clear of reminders of the trauma and detaching emotionally from others; and hyperarousal that causes sufferers to startle easily, sleep poorly and be on alert for potential threats. These problems must last for a month or more for someone to qualify for the PTSD label.

Immune to Trauma?
After the terrorist attacks of September 11, 2001, many mental health experts confidently predicted an epidemic of PTSD, especially in the most severely affected locations: New York City and Washington, D.C. The true state of affairs was much more nuanced, however. It is certainly true that many Americans experienced at least a few post-traumatic symptoms following the attacks, but most of the afflicted recovered rapidly. In a 2002 study psychologist Roxane Cohen Silver of the University of California, Irvine, and her colleagues showed that about 12 percent of Americans suffered significant post-traumatic stress between nine and 23 days after the attacks. Six months later this number had declined to about 6 percent, suggesting that time often heals the psychic wounds.

Work by epidemiologist Sandro Galea of the New York Academy of Medicine and his colleagues, also published in 2002, revealed that five to eight weeks after 9/11, 7.5 percent of New Yorkers met the diagnostic criteria for PTSD; among those who lived south of Canal Streetthat is, close to the World Trade Centerthe rates were 20 percent. Consistent with other data, these findings suggest that physical proximity is often a potent predictor of stress responses. Yet they also indicate that only a minority develops significant post-traumatic pathology in the aftermath of devastating stressors. Indeed, the overall picture following the 9/11 attacks was one of psychological resilience, not breakdown.

More broadly, research that psychologist George A. Bonanno of Columbia University and his colleagues reviewed in 2011 suggests that only about 5 to 10 percent of people typically develop PTSD after experiencing traumatic life events. And although the rates rise when stressors are severe or prolonged, they hardly ever exceed 30 percent. The rare exceptions may occur with repeated trauma. In another 2011 study psychologist Stevan Hobfoll of Rush Medical College and his colleagues reported that of 763 Palestinians living in areas rife with political violence, more than 70 percent exhibited moderate PTSD symptoms and about 26 percent had severe symptoms.

The finding that PTSD is not an inevitable sequela to trauma has spurred investigators to pursue factors that forecast relative immunity to the condition. Across studies, higher income and education, strong social ties and male gender tend to confer heightened resilience, although these predictors are far from perfect. People who usually experience very little anxiety, guilt, anger, alienation and other unpleasant emotionsthat is, who have low negative emotionalityare also less likely to suffer from PTSD following trauma. Thus, in ways that researchers do not yet understand, individual characteristics must combine with trauma to produce this illness.

Emotional Triggers
Not only is trauma insufficient to trigger PTSD symptoms, it is also not necessary. Although by definition clinicians cannot diagnose PTSD in the absence of trauma, recent work suggests that the disorders telltale symptom pattern can emerge from stressors that do not involve bodily peril. In 2008 psychologist Gerald M. Rosen of the University of Washington and one of us (Lilienfeld) reviewed data demonstrating that significant PTSD symptoms can follow emotional upheavals resulting from divorce, significant employment difficulties or loss of a close friendship. In a 2005 study of 454 undergraduates, psychologist Sari Gold of Temple University and her colleagues revealed that students who had experienced nontraumatic stressors, such as serious illness in a loved one, divorce of their parents, relationship problems or imprisonment of someone close to them, reported even higher rates of PTSD symptoms than did students who had lived through bona fide trauma. Taken together, these findings call into question the long-standing belief that these symptoms are tied only to physical threat.

In light of these and other data, some authors have suggested that the PTSD diagnosis be extended to include anxiety reactions to events that are stressful but not terrifying. Yet such a change could lead to what Harvard University psychologist Richard J. McNally calls criterion creepexpanding the boundaries of the diagnosis beyond recognition. This and other controversies aside, recent results raise the possibility that PTSD is a less distinctive affliction than originally thought and that its symptoms may arise in response to a plethora of intense stressors that are part and parcel of the human condition.

This article was published in print as "When Coping Fails."