Each year in the United States, approximately 45,000 people kill themselves. There’s little data on how many of these suicides occur in public view, and even less on how many people witness them when they do. One study analyzed all completed suicides in Riverside County, California, from 1998 to 2001, and estimated that around 17 percent took place in public places, like roads, railways, and fields. Another study, from 1994, reviewed forensic reports of 1,183 suicides among people affiliated with the U.S. Air Force and found that 4 percent were committed in the presence of at least one other person.

Ashley Tate Hatton was studying for her Ph.D. at the California School of Professional Psychology when she saw the controversial documentary The Bridge, about people who leap from San Francisco’s iconic Golden Gate Bridge. Watching the victims fall—even on camera—Hatton felt queasy, complicit. When it came time to choose a subject for her dissertation, she decided to study the effects of witnessing suicide in real life. She posted ads around campus and online, and to her surprise, soon found a small group of people who had seen strangers take (or attempt to take) their own lives.

“I thought it was a long shot,” she says. She hadn’t realized how common an experience it was. “I didn’t have to travel outside of Southern California—I was prepared for that.”

Three of Hatton’s subjects had seen people jump from bridges, three from a building; two had seen people shoot themselves; three had seen people step in front of vehicles. One of the subjects, a man in his 50s, was waiting for a bus when a young man threw himself in the path of an oncoming van. For the next several days, the onlooker thought about it constantly. He became obsessed with the precariousness of life, and told Hatton that he began to feel as though “every second could be [his] last.” When she met him three years later, she found that he no longer ruminated incessantly about the memory, but he still dreamed about it from time to time. He told her he had become a more cautious driver; he worried about running someone over.

All but one of Hatton’s subjects said that they considered the experience traumatic, and one, according to Hatton, met the criteria for chronic post-traumatic stress disorder. (Those who had been more involved—who had called 911 or tried to talk the victim down from the ledge—tended to be more affected.) Nine of the 10 said that pictures about the event popped into their minds; six admitted they thought about it without meaning to; three had physical reactions when they were reminded of the event, including sweating, nausea, and trouble breathing. Eight said that the experience had a significant impact on their lives, including one who started volunteering at the Red Cross, and two who resolved not to act on their own suicidal fantasies.