A century ago, we called it shell shock—the legacy of trauma that wrought havoc on the bodies and minds of some World War I veterans. Today, when survivors of terrible events experience flashbacks and fears that disrupt their daily lives, long after the actual threat is gone, it is called post-traumatic stress disorder, or PTSD. Whatever you call it, we now know that direct exposure to extreme deprivation, violence, dislocation or torture can transform not only those who experience it but also their future offspring.

Scores of studies have shown that the adult children of Holocaust survivors who suffered from PTSD are at high risk of developing the disorder themselves. Now, 25 years after the genocide against the Tutsi of Rwanda, an unusual collaboration between Rwandan and Israeli researchers has uncovered similar intergenerational effects. The atrocities witnessed by surviving Tutsis have left enduring marks on their adult children, the vast majority of whom hadn’t been born when the genocide took place in 1994.

The study, published last month in the journal Psychiatry Research, was led by Amit Shrira, a professor of psycho-gerontology at Bar Ilan University, working with Benjamin Mollov, a Bar Ilan social scientist, and Chantal Mudahogora, a Canadian researcher and a survivor of the genocide in Rwanda. The team studied 60 pairs of Tutsi survivor parents and their adult children, all of whom completed two questionnaires about their psychological state.

Not all families who share a brutal history are the same, the researchers discovered. According to their responses, the Tutsi parents could be divided into three groups.

The first group (33.3%) suffered from complex PTSD, a new diagnostic category that includes panic attacks, recurring nightmares and intrusive memories. They perceived themselves as helpless and had difficulty maintaining close relationships. The second group of parents (26.7%) had simple PTSD: They continued to relive the traumatic events and personal losses they had experienced and were haunted by a sense of threat. The third group (40%) didn’t have any clinical symptoms of PTSD. Though it is impossible not to experience distress after surviving a genocide, this group of parents seemed to be surprisingly resilient; their feelings of grief didn’t disrupt their lives to the degree that they merited a clinical diagnosis.