This is an excerpt from the new book Origins: How the Nine Months Before Birth Shape the Rest of Our Lives, by Annie Murphy Paul.

At 8:46 AM on September 11, 2001, there were tens of thousands of people in the vicinity of the World Trade Center—commuters spilling off trains, waitresses setting tables for the morning rush, brokers already working the phones on Wall Street. About 1,700 of these people were pregnant women. When the planes struck and the towers collapsed, many of these women experienced the same horrors inflicted on other survivors of the disaster: the overwhelming chaos and confusion, the rolling clouds of potentially toxic dust and debris, the heart-pounding fear for their lives. 

As the catastrophe began to unfold, psychiatrist Rachel Yehuda was arriving for work at the Bronx Veterans Affairs Medical Center, about 15 miles north of the Twin Towers. “I was leading a meeting at the center when I got a call from my mother, who lives in Florida,” Yehuda tells me. “She had seen news of the attack on TV and wanted to know if I was all right.” Yehuda and her colleagues located a television of their own and watched, aghast, as the awful events of that day took shape. “Of course I was thinking about what the long-term reactions of the survivors would be,” she says. Yehuda, who is director of the Traumatic Stress Studies pision at the VA center and a professor at the Mount Sinai School of Medicine in Manhattan, is a leading expert on post-traumatic stress disorder, a condition that forces survivors of a traumatic event into a state of hyper-vigilance, assailing them with nightmares and panic attacks. In the course of her career as a PTSD researcher, she has worked mostly with Holocaust victims and Vietnam War veterans—people whose trauma happened far away and many years, even many decades, ago. As Yehuda watched in real time as tragedy struck her own city, she was already thinking about how to investigate its impact. 

In the years since 9/11, Yehuda has coauthored more than a dozen articles about its effects on survivors, including several deeply suggestive studies of women who were exposed during pregnancy. “I wanted to look at this population in particular because I have a long-running interest in the transgenerational transmission of PTSD risk, or the handing down of a susceptibility to PTSD from parent to child,” she explains. Yehuda encountered a vivid example of this phenomenon in 1993, when she opened the first clinic in the world devoted to the psychological treatment of Holocaust victims. She expected a flood of inquiries from people who had experienced Nazi persecution firsthand. But she soon got a surprise: for each call her clinic received from a Holocaust survivor, it was getting five calls from their grown children. “Many of these members of the second generation had symptoms of PTSD,” Yehuda says. They reported the same nightmares, the same panics, the same hair-trigger vigilance their parents had. Yehuda’s research confirmed that the offspring of parents with PTSD were more likely to develop PTSD themselves, even though they were no more likely to encounter traumatic events than other people. 

How could this be? The traditional, psychoanalytically inflected explanation was that the younger generation was sensitized to trauma by growing up around survivor parents—hearing their stories, observing their struggles, enduring their silences. In the words of one commentator, the children of Holocaust victims bore “the scar without the wound.” Yehuda, too, initially subscribed to this notion. “I was really convinced that the early experiences of the child, being bombarded for years by the psychiatric symptoms of the parents, accounted for the transgenerational effect we observed,” she tells me. But in the years after her clinic opened, she began to wonder if some other influence was at work, perhaps even before birth.

Yehuda’s previous research had found that low baseline or “basal” levels of cortisol were a marker of vulnerability to PTSD: people with low basal levels of cortisol are more likely to develop PTSD following a traumatic event. Cortisol is a hormone released when the body is under stress; one of its functions is to stop the stress reaction once it has run its course. In people with low levels of cortisol, the body’s heightened state of alarm, so necessary in the midst of a crisis, does not subside once the crisis is over. Yehuda’s research had also established the striking fact that the offspring of people with PTSD have low basal cortisol as well. Is this commonality simply genetic? Or could it be passed down in utero? Nine-eleven provided an opportunity to find out.