Chanrithy Him was employed to study stress among Cambodian refugees. But the job awakened memories of her own experiences in labour camps under a death regime. Tim Cornwell reports.

In the early 1980s, teachers at an Oregon high school began to notice unusual behaviour among their Cambodian refugee students, including severe "startle reactions". The film The Killing Fields had not yet been released, and many Americans had yet to grasp what these teenagers had witnessed. One girl, digging in the ground at a teacher's home, unearthed a bone and began screaming. Others began talking about incidents of cannibalism. It was then that a teacher approached psychiatrists at Oregon Health Sciences University.

Chanrithy Him, then in her late teens, was one of the children interviewed by the Oregon researchers in what would become an extended study of refugees who fled the murderous regime of the Khmer Rouge in Cambodia in the 1970s. In 1989, Him became a researcher at the university, acting as an interpreter and interviewer in follow-up studies to gauge the long-term incidence of post-traumatic stress disorder (PTSD). Still recovering from her own horrific experiences - the loss of most of her family, near starvation, threatened execution, witnessing gruesome killings - Him was to listen, professionally, to scores of similar stories. "As a researcher, my job was to be a cultural voyeur," she writes in her forthcoming book, When Broken Glass Floats. "I was to use my knowledge of Cambodian customs, culture and my own wartime experiences to establish a common ground with other refugeesI In conducting psychiatric interviews, I was both the insider, who knew their trauma, and the outsider, the dispassionate, clinical researcher." She was to listen, record answers and press questions, up to and beyond the point where people broke down. She was, she said, "the mirror of her results"; the stress symptoms she analysed were her own.

Last week, in a comfortable detached home on the hilly outskirts of Portland, Oregon, Him revisited one of the fellow refugees who are her "subjects". The 39-year-old mother of four and hospital clerk explains politely that "she's blocked out quite a bit. I can never remember that much to be honest." Moments later, she is weeping.

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The Killing Fields, she says, was "only half of it". At the age of 11, she was trained by the Khmer Rouge as an informant, creeping under stilt houses at night to eavesdrop on families, some of whom were later massacred. She bears the stigma in the knowledge that her parents, three sisters and a brother were killed, and that her sister lost a husband and three of four children to the Khmer Rouge. The "blocked-out" memories are all too vivid. She watched children being tossed in the air and bayoneted. "I knew nothing, I was just a kid," she says. "They killed in front of me, I was numb. I didn't care, I didn't do anything. I'm in control, I'm one of them - that's how I felt." Today, the sight of a military vehicle, of someone wearing black or the night cries of her own children kick her back to Cambodia. She has talked to her 11-year-old son about his missing grandparents, but the rest of the story will wait.

The 12-year follow-up study of Khmer youngsters from Portland, published last year, revealed that a third still suffer post-traumatic symptoms, the same percentage as six years before. Yet the results also show a community of survivors: five out of six are working, only a couple of people on welfare, almost no abuse of alcohol or drugs. Statistically, the Cambodians have fared far better than US Vietnam combat veterans. "Given the horrific war trauma these subjects experienced over an almost four-year period, it is surprising that the prevalence rates of PTSD were not 100 per cent," the study concludes. "The capacity of these subjects to transcend their suffering and become productive adult citizens is a tribute to their courage and endurance and shows that prior war trauma, in childhood, need not be incapacitating in its long-term impact."

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But beneath this surface success lies profound damage. Veronica Ngi, 26, another of Him's interviewees, works for a financial services company. She remembers foraging as a child alone in a forest, eating whatever she could find on the ground and dodging bombs and bullets. She entered counselling in 1994 and now runs marathons, because "the emotion was too painful, if I didn't release it, it would drive me insane".

Him's book, a compulsive labour over ten years, will be published in April on the 25th anniversary of the Khmer Rouge's taking power in Cambodia. It tells the Cambodian story to a new generation - and this time, through the eyes of a young girl, not those of western journalists.

Him majored in biological sciences at the University of Oregon. She had hoped to go to medical school, but fulfilled a different ambition in writing the book. She wrote to David Puttnam, the producer of The Killing Fields, proposing her story as a film, in which she dreams of playing her mother.

"This book," Him says, "is a monument to my parents, my siblings, and the many Cambodians who died." Although inspired in part by her role as a researcher, the book is primarily the story of Him's childhood. It provokes fresh outrage over the Khmer Rouge atrocities, but Him avoids pointing fingers at other possible culprits - the Vietnamese or America itself. "I want to tell the dark side of human nature," she says, "to give a voice to the children of Cambodia during that time."

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Him produced an early chapter of the book for a writing class. It described how she took her three-year-old brother, now a Mormon student at Utah's Brigham Young University, for a final visit to their mother, who lay dying in a Khmer Rouge "hospital" in conditions so appalling that patients would trap rats to eat. Her teacher gave her a B+ for the story, and added: "How can I give you a grade when I can't stop crying?" As she wrote her book in pastoral Oregon, she says, she was transported back to Cambodia. "Why am I compelled to write this book, if it takes going close to insane?" she asked herself. "Once I finished the stories, I cried less and less. That recognition of the healing process I was able to share with some of the subjects."

An estimated 150,000 Cambodians reached the United States in the aftermath of the Khmer Rouge killings, most, like Him, via refugee camps in Thailand and the Philippines. Those most determined to make a new life in the US may be the most reluctant to look back; therapy is likely to take second place to hard work.

When Broken Glass Floats takes its title from a Cambodian proverb about what happens when good and evil are thrown together in the river of life. Him heard the story from her older sister, Chea, who took over the maternal role in the child labour camps before perishing of disease and malnutrition. Good, symbolised by a kind of squash, will float; evil, represented by broken glass, will sink. In Cambodia under the Khmer Rouge, from 1975 to 1979, broken glass floated.

War came into Him's comfortable life in Phnom Penh, where her father worked as a government trade official, in 1969, with the Viet Cong incursions and the secret bombing of Cambodia under President Richard Nixon. As a four-year-old, when the ground shook, Him heard her father talk of the "B-cinquante-deux"; only years later in the US would she recognise this as the B-52 bomber. Some of the book's most bittersweet passages concern her relationship with her father, who took her to restaurants and the royal palace, and for whose scooter she would listen for at the end of the day. As Phnom Penh swells with war refugees, as the killing moves closer, he persists in going to his deserted office, "his desperate bid to be normal" for his children. She rides on his scooter to their old home, and finds it a bombed-out, violated wreck. The young Him is shocked by the soldiers' bad manners. Not long after, her father is taken away in an oxcart for "orientation".

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The book paints Cambodia under the Khmer Rouge, which took power when Him was nine, as a vast concentration camp, where scrounging a mouthful of solid rice, with a diet of crickets and silkworms, was "like going to heaven". Part of Him became an observer of this tragedy, the watchful child, feeling the pain of others amid her personal struggle to survive.

As a researcher, Him interviewed about 50 people in the first study she worked on, and about 40 in the follow-up. Eventually she could no longer write-up her research; it was too painful. In 1997, she went back into therapy herself. "The subjects had unloaded their stories to me," she said. "I was like a super-saturated sponge. I needed to download that to another person."

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Chanrithy Him's When Broken Glass Floats: Growing up under the Khmer Rouge will be published by Norton on April 17.