In 1991, Karestan Koenen was a recent college graduate and Peace Corps volunteer who arrived in a village in Niger eager to help local women start small businesses. When her sister came to visit during Christmas, the two decided to travel north to Agadez, a city in the Sahara.

There, on the morning of Dec. 27, two male traders stopped by, trying to sell them jewelry. Koenen’s sister went to the market with one of men to have a look. While she was gone, the second man grabbed Koenen, held her down and raped her.

Traumatized by the experience, Koenen was medically evacuated to the United States two days later and resigned from the Peace Corps. She returned to New Jersey to live with her parents, but the assault continued to haunt her. Increasingly, she became depressed.

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A psychologist diagnosed Koenen with post-traumatic stress disorder, or PTSD, a condition triggered by a traumatic, scary or dangerous event, and, for reasons still unclear, seems to disproportionately afflict women. These assaults can include combat, sexual assault, gun violence, accidents, natural disasters, even the death of a loved one.

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“I lay in bed, unable to sleep, thinking of ways to kill myself,” she recalls. “When I did sleep, I had nightmares. I lost interest in everything. I couldn’t read and was too jumpy to sit through a movie or watch TV. I was irritable with my family. I was always on guard — angry — and couldn’t stop thinking about what had happened. I felt like I was stuck in a dark tunnel, moving more and more quickly, but it only got darker.”

Symptoms can last for years if untreated. These include flashbacks (reliving the event), nightmares, avoidance (shunning settings that remind them of the trauma), nervousness or jittery feelings, and feelings of shame and guilt.

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PTSD received national attention in the 1970s when many Vietnam veterans returned home with symptoms. But experts already knew about the emotional toll of combat, having seen it in veterans of both World Wars. They called it “shell shock.”

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Then, it was considered a disorder of war-weary men. It still is, but the trauma extends beyond the violence of war — and beyond men. Today, in fact, women — both civilians and women from the military — are more than twice as likely as men to suffer from PTSD. It affects nearly 10 out of every 100 women — compared with fewer than 4 out of every 100 men, according to the Department of Veterans Affairs National Center for PTSD.

“It is an issue for girls and women across their life span,” says Janine Clayton, director of the office of research on women’s health at the National Institutes of Health.

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“Women and men experience different types of trauma and at different times in their lives. Men experience violence-related traumas, often at an older age, while women are likely to experience sexual assaults and sexual abuse at a younger age, when the brain is developing, and they are at increased risk for PTSD later in life,” she says. “The timing is different, the nature of the trauma can be different, and how the brain responds can be different. It’s normal to feel symptoms, but it’s not normal for them to persist.”

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A family friend urged Koenen to seek professional help. She began seeing a psychologist, who encouraged her to talk about her experience. “Going to therapy was the turning point,” Koenen recalls. “She said that the only way to get better was to talk about what happened. So we did, slowly.”

Over time, Koenen recovered. Moreover, the experience ultimately propelled her into a research career with a focus on trauma and PTSD, in part to “help other women who had gone through what I had,” she says.

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There has been increasing public attention paid in recent years to sexual assault, in part because of the #MeToo movement and growing scrutiny of public figures that began two years ago. The data on rape is especially dramatic. A recent study of women ages 18 to 44 found that more than 3.3 million women’s first sexual experience was rape, a number the researchers said was probably conservative.

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“The statistics are really grim regarding PTSD following rape,” says Tracey Shors, professor of behavioral and systems neuroscience at Rutgers University. “Rape is the most likely trauma to induce PTSD. The closer you are physically to trauma, the more likely it will cause PTSD symptoms — and you can’t get any closer than rape.”

Women in the military are vulnerable to twofold trauma, combat and sexual assault. PTSD has been rising among service women, in part because of more reporting and more women who join, according to Tara Galovski, director of the women’s health sciences division at VA’s PTSD center. An estimated 48,000 women — from the 455,875 female veterans who use VA health care — are being treated for PTSD, she says.

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“Veterans often report exposure to both types,” that is, combat and sexual assault, Galovski says. “Further, traumas that occurred prior to enlistment or deployments also increase the risk. Exposures to multiple traumas across the life span increase the risk.”

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Not everyone who experiences trauma develops PTSD; still, about 8 million adults — both men and women — have it during a given year, according to VA. Many struggle in isolation, either unaware that help is available or reluctant to seek it.

PTSD also raises the danger of heart disease, obesity, diabetes, blood clots, certain cancers and autism in the offspring of survivors. This affects men and women, although much of the recent research has been conducted on women.

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“For many years, PTSD was understood to be strictly a mental disorder, but evidence has been mounting that PTSD also has physiological effects,” says Andrea Roberts, a research scientist at the Harvard T.H. Chan School of Public Health, an investigator in these studies. “It has effects on our bodies that increase the risk for a broad array of diseases.”

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The reasons it afflicts more women than men are unclear, although studies are underway to figure them out. Koenen, a professor of psychiatric epidemiology at Chan, thinks the distinction, in part, may relate more to the type of trauma women experience, rather “than being male or female.”

Women are more likely than men to be sexually assaulted, but experts think there is more to it than that. They think multiple factors may be involved, including hormones, brain stress responses and genetics. Also, they point out that women are more susceptible to depression, anxiety and other mental health issues.

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Miranda Olff, a psychiatrist with the ARQ National Psychotrauma Center in the Netherlands, says many women experience interpersonal trauma — such as rape or sexual abuse — often at a younger age, when it can interfere with psychosocial and neurobiological development.

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“We know very little about the female brain,” Olff says. “There is no male or female brain. But there is some evidence of increased activity of brain regions that have to do with, for instance, emotional processing. Women respond more strongly with their stress hormone system relative to men, while men show higher levels of hyperarousal symptoms, like anger or aggression.”

Farris Tuma, who directs the traumatic stress research program at the National Institute of Mental Health, agrees. “Under stress, men and women interpret what they see differently,” he says. “Specifically, how our brains process emotions is different. Women show enhanced connectivity or coordination in the emotion processing parts of the brain, and this is reduced in men under stress. The implication of this is that women may be more efficient in incorporating the trauma, and linking that experience with very strong fearful, terrifying emotions — making it harder to recover.”

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Women also may have more trouble than men “unlearning” fear, a process known as “fear extinction,” says Edna Foa, professor of clinical psychiatry at the University of Pennsylvania’s Perelman School of Medicine.

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“Women have a deficit in fear extinction,” she says. “That way, they are more likely to maintain PTSD symptoms.”

Studies also indicate that sex hormones may play a role. Foa is testing the effects of estrogen on fear extinction, believing when estrogen levels are high, women can more quickly eliminate fear.

Genetics, too, may influence the development of PTSD. Koenen is leading a large study seeking to identify genetic differences among people, both men and women, who have PTSD and those who don’t.

In a first step, the researchers reported a strong genetic component to PTSD, identifying six genomic regions associated with the disease, although these differences were present only in men. The scientists are continuing to search for other genetic clues, including those specific to women.

“If we see differences with PTSD vs. those without PTSD, that might point us to important biological clues,” she says. “If we can better understand the biology of the disorder, it could lead to new treatments.”

There are effective treatments for PTSD that include cognitive processing therapy (CPT) and prolonged exposure (PE), both widely used by the military and elsewhere.

With PE, for example, which was developed by Foa, patients gradually return to settings they’ve been avoiding — for example, going out after dark — and confront their painful memories, which can restore forgotten details. Among other things, the latter often helps women shed feelings of self-blame, Foa says.

“I had a patient who was raped by a boyfriend and his friends,” she recalls. “She wouldn’t let herself think about what happened. By revisiting the memory, she discovered she fought them like a lioness. She realized it wasn’t her fault, and her sense of guilt and shame went down.”

No one should resist treatment because of these emotions, experts say. “It’s important to understand that this is not about a moral failing or character,” Clayton says. “PTSD is an illness. If you had a broken leg, you’d put a cast on it. You don’t need to be strong and walk on. This is something that can and needs to be treated.”