When researchers look at trauma, they don’t just look at the lives of those who experience traumatic events. Often, they turn their attention to offspring, because numerous studies, some conducted on the children of Holocaust survivors, have shown that parents can pass symptoms of trauma onto their kids. This is called "intergenerational trauma," and although it’s been described in academic literature, researchers still don’t fully understand how it occurs. One idea is that symptoms like anxiety and depression can be passed down through changes in gene expression in sperm. But what of specific fears, like a fear of dentists? How do they make their way into the psyche?

According to a new study, published today in Proceedings of the National Academy of Sciences, the mechanism might be scent — at least when the fear occurs in mice. "We found that the mother that expresses fear in the presence of their newborn pups passes her fear to her pups through scent," says Jacek Debiec, a psychiatrist at The University of Michigan and a co-author of the study. "And these fears are long-lasting."

In the study, Debiec and his team exposed adult female mice to the smell of peppermint, a "neutral" scent, while giving them mild electric shocks. This produced long-term fear response in the female mice that researchers were then able to trigger using peppermint. Then, the researchers matched the mice with males who hadn’t undergone the fear conditioning, so they could mate.

The peppermint triggered a fear response in the pups too

"Once the pups were born, we exposed them and their mothers to the peppermint odor," Debiec says. The fear that the mothers expressed in the presence of the pups helped the newborns learn to fear peppermint as well. And eventually, Debiec says, "it also triggered fear in the pups when the mother wasn’t around."

But that experiment alone wasn’t enough to explain how the mothers were transmitting fears onto their pups. So, the researchers separated the mothers from their pups so that only their scent could reach the pups. This made sense, because newborn mice are "underdevelopped" compared to human babies, Debiec says, meaning their eyes and ears aren’t fully functional. And the experiment worked: the pups learned to fear peppermint even when they couldn’t see or hear their mothers — and even after they were weaned.

"Once they learned the fear response through scent, the pups avoided the peppermint odor," Debiec says, or froze in fear. Moreover, to make sure that changes in gene expression in the mother weren’t contributing to the fear transmission, the researchers used foster mothers. Even when the mothers weren’t related to the pups, the results were unchanged.

Even when the mothers weren’t related to the pups, the results were unchanged

"Newborns don’t really make much of their own experience in their surrounding environments," Debiec says. "But fear responses can still be passed down." This might seem like a huge disadvantage for the pups, but it’s actually considered a fairly efficient way of learning about the world. "It’s ecologically important that pups acquire information about fears outside of the nest before by being exposed to [the mother’s] fear before they leave it," he says. The problem only arises when this also applies to things like PTSD or a fear of dentists, Debiec says, "because it’s exaggerated."

The researchers think that the association between the fear response and the peppermint occurs through elevated blood corticosterone, a hormone that’s equivalent to human cortisol. "We found that exposure to the frightened mom significantly elevated these levels," Debiec says.

conducting a similar study in humans wouldn't be ethical

It’s too early to know how this experiment applies to humans, because it’s still unclear to what extent humans use chemicals to communicate with each other. And conducting a similar study in humans isn’t feasible because the ethics behind it would be extremely questionable. Still, Debiec says, the idea that a mother can pass her fears down to her offspring in a long-term fashion is important.

"Learning is usually short-lived in human and non-human infants; it’s what we can ‘infantile amnesia,’" Debiec says. As a result, newborn mice rarely remember what they learn when they’re in a pre-weaning state. But, similar to fears that arise after a child experiences abuse, the fear response triggered by the peppermint persisted — something that Debiec says points to how powerful parental influence can be.

Rachel Yehuda, a psychiatrist at Mount Sinai Hospital in New York who did not participate in the study, told The Verge in an email that Debiec’s findings are "very interesting," but she didn’t find them altogether surprising. "Those of us who have studied populations such as adult children of Holocaust survivors have seen evidence of, and have attempted to describe, this kind of transmission in the clinical arena," she said. Still, she said, the study is valuable because it includes the type of molecular analysis that would not be possible in living human brains.

it took the field a while to even believe this was a legitimate phenomenon

"It took the field a while to even believe this was a legitimate phenomenon," Yahuda said, but the science of intergenerational transmission is now increasingly accepted. This is a good thing, she said, because identifying the brain changes that result from such transmission could play a crucial role in helping people understand the impact of parental experiences. "Your fears are not only a response to your personal challenges," Yahuda said, "but those that your parents had as well."