A watch stopping at 8:15, which is the time of the explosion time of the atomic bombing of Hiroshima, displayed at Hiroshima Peace Memorial Museum on July 30. (Kimimasa Mayama/EPA)

World War II left a huge number of people traumatized. They often carried physical and mental scars that proved difficult, if not impossible, to fully heal over time.

But the trauma also lives on in people who were not even born when the events of World War II occurred. These are the children or even grandchildren of the era's survivors, and the events still resonate in their lives, creating a deep cloud that can last a lifetime.

This week, a Scottish group called Never Again Ever! started a campaign to help support the grandchildren of Holocaust survivors. The group's founder, Dan Glass, a 29-year-old Londoner, grew up with his father's stories of the Holocaust, which had been passed on to him by his own parents who had survived it. Glass said it left him with a heavy burden.

"My father had a whole wall of books on the subject of the Holocaust – it was all he wanted to talk about, but it was so harrowing for me," Glass told the Guardian. After researching how other grandchildren of Holocaust survivors dealt with their feelings about that horrifying time, he realized he wasn't alone in finding it hard to cope with. “I have been privileged to hear so many stories from young people who should now be able to live with joy – but their lives are damaged and they weren't even there," Glass said.

Such an idea isn't limited to the Holocaust. This week marks the 70th anniversary of the nuclear attack on Hiroshima, Japan. As many as 140,000 people were killed in that city, with almost 80,000 killed in a second attack in Nagasaki three days later. Many more, however, lived through the attacks: More than 183,000 survivors of the attack are still alive today, still carrying the painful memories of what happened on Aug. 6, 1945.

Last August, the New Yorker's Sarah Stillman traveled to Ohio to hear not only the story of Tomiko Shoji, a Hiroshima survivor, but also that of Keni Sabath, Shoji's granddaughter. Shoji explained how the bombing had not only brought about terrible physical problems, including an early loss of her eyesight and hearing, but also deep emotional problems that she can't shake decades later. She felt shame due to prejudice against those who lived through the post-bombing radiation, for example, and irrational fear whenever she saw a plane overhead or heard thunder.

What's really remarkable is that those feelings didn't stay restricted to Shoji's generation. The trauma spread through two generations to Sabath, who told Stillman that after she discovered what her grandmother lived through, she became anxious and would panic whenever she saw an airplane overhead, just like Shoji did. “[My parents] thought I was haunted by the ghosts of Hiroshima," she explained.

The idea that you can be traumatized by something you didn't personally experience may strike some as far-fetched, but there's a body of academic literature that supports the idea. The phrase "secondary traumatisation" was coined to describe the trauma passed on by victims to their children. "Transgenerational trauma" can be used to describe it when the trauma is felt by future generations.

Studies claim to have found evidence that the trauma is being passed from Holocaust survivors to younger generations. One found that Israeli soldiers who fought against Lebanon in the 1980s were more likely to develop Post Traumatic Stress Disorder if their parents had been Holocaust survivors. Another showed that the grandchildren of Holocaust survivors were overrepresented by 300 percent at a psychiatric clinic. Some research even suggests that there is a physical component to the trauma – with it affecting breast milk of survivors or even altering the DNA of their descendents.

If this is all really true, think of the huge effect transgenerational trauma would have on the world. For American readers, it might be closer to home than you think. PTSD is relatively common among U.S. military veterans, but some believe that it might be actually be contagious, attaching itself to their spouses and children, too. If the trauma can spread across generations, that would just be the start: Consider the immense burden transgenerational trauma have placed on African Americans and Native Americans. Expand it further to the world stage, and the effects of transgenerational trauma would be enormous. The horrors of World War II and Nazi Germany are rightfully well-known, but they were far from the only horrors in the 20th century that could have left imprints on future generations. The impact of such trauma is unquantifiable.

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In the end, if this trauma really does exists so widely, the big question is what can be done about it. The Scottish group Never Again Ever! suggests switching from what it says is "melancholic memorialisation" to "positive action," which includes things, such as mental health treatment and political campaigning. Something similar is happening in Hiroshima, where the dwindling numbers of survivors hope to pass on their memories to younger people so that the lessons learned won't be forgotten.

As Emiko Okada, a 78-year-old survivor of the Hiroshima attacks, explained this week: “We don’t want you young generations to go through what I did."

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