Medical researchers are beginning to ask a question: Can the effects of a trauma experienced by one generation somehow be passed on to subsequent generations? Could the deeply traumatizing experience of surviving, for example, genocide or severe malnutrition negatively impact the health of subsequent generations of survivors’ children?

Jews, homosexuals, and others experienced brutal persecution during World War Two. Contemporary Native Americans are the offspring of survivors of a concerted effort at genocide, both physical and cultural, through the Indian Wars of the nineteenth and twentieth centuries, and the boarding school system that stripped Indigenous North Americans of their cultural knowledge, including their languages and medicine ways. Black Americans, too, suffered through geographical dislocation when their ancestors were transported from their homelands to work on plantations in the Americas, as well as relentless oppression through racism and American apartheid.

Researchers, according to an article in the American Psychological Association Monitor, “are exploring the intergenerational effects of the Holocaust, the Khmer Rouge killings in Cambodia, the Rwandan genocide, the displacement of American Indians and the enslavement of African-Americans. The transgenerational effects are not only psychological, but familial, social, cultural, neurobiological and possibly even genetic as well, the researchers say.”

While hotly debated among social scientists and medical researchers, there is growing evidence that the trauma of one generation may indeed negatively impact later generations.

Washington State University associate professor of nursing Lonnie Nelson says that “Stressors experienced by parents around the period of conception and during pregnancy do have an effect on DNA methylation patterns that get expressed in the offspring. How those changes in methylation patterns manifest as physical health conditions has yet to be established. We don’t know exactly how those changes alter the gene expression. We just know they’re there and have the potential to affect expression. Like everything in [biology] it’s not nature or nurture, it’s both.”

DNA methylation mechanism that alters the function and expression of a gene. Heritable changes in gene expression were long thought to be exclusively the result of a change in the underlying DNA sequence, as when genotypes combine during sexual reproduction. Methylation, though, “tags” DNA molecules in ways that can turn them on or off. This is called epigenetic change, and such changes occur within a living organism due to psychological or environmental stress (due to exposure to certain chemicals in plastics, for example), age, and disease. Epigenetic changes can affect not only the organism during life, but its offspring, as well.

A recent, widely reported study found that survivors of Confederate prison camps during the American Civil War were impaired: they suffered from what we would now call post-traumatic stress disorder. Crowded together (with just a few square feet per person in some prisons) and drastically malnourished, these survivors, mostly men, had a shortened life expectancy compared to the general population at the time. That paternal trauma was apparently passed on to the men’s children, whose sons (but not their daughters) also experienced a shortened life span. The researchers write that while they “cannot rule out fully psychological or cultural effects, our findings are most consistent with an epigenetic explanation.”

The study of POWs involved thousands of survivors. A much smaller study on Holocaust survivors detected a change to a specific gene in both survivors and their children. While that study has been criticized for its small sample size, a similar study of the children of Holocaust survivors living in Brazil found similar effects.

Other studies of the effects of historical trauma on humans has involved Australian Vietnam War veterans, whose children were more likely to suffer from family dysfunction and the PTSD-like symptoms also suffered by their parents; Offspring of torture victims likewise showed increased symptoms of anxiety, depression, and other PTSD-like symptoms.

Research on mice has started to zero in on the genetic mechanisms of trauma inheritance.

In one experiment, researchers trained male mice to fear the scent of cherry blossoms. Acetophenone, which smells like cherry blossoms, was blown through the cages of mice while, at the same time, the received a non-lethal electrical shock to their feet. After a few repetitions, the mice associated the scent with pain. When the male mice mated, their second and third generation offspring retained that association. The dissected brains of offspring had a greater number of receptors for that particular scent. Sensitivity was linked to a epigenetic change on a smell receptor in the parent mice.

Some researchers insist that such methylation events are rare, as the DNA slate is ostensibly wiped clean when a sperm combines with an egg. But a growing number of studies indicate that in fact genetic changes are carried forward, possibly via histones or RNA in sperm.

A 2018 article in Brain Sciences offers a review of all current literature on the matter, and concludes that “Our review found an accumulating amount of evidence of an enduring effect of trauma exposure to be passed to offspring transgenerationally via the epigenetic inheritance mechanism of DNA methylation alterations and has the capacity to change the expression of genes and the metabolome.”