Babies smell their mothers' fear to learn what to be afraid of

Mothers may emit odors that teach their babies what to be afraid of, even if the fearful experience is one that the baby has never been exposed to, according to a study conducted by researchers from the University of Michigan and published in the Proceedings of the National Academy of Sciences.

The research may help explain a phenomenon that has perplexed scientists for generations: Children may have intense trauma reactions to events that they never experienced, but that their parents did. For example, children of Holocaust survivors often exhibit nightmares, flashbacks and avoidance behaviors associated with their parents' experiences, even if those happened before they themselves were born.

Adopting parental fears
According to lead researcher Dr. Jacek Debiec, who has studied grown children of Holocaust survivors, these reactions seem too deeply rooted to be the result of simply having heard stories about the frightening events.

"Our research demonstrates that infants can learn from maternal expression of fear, very early in life," Dr. Debiec said. "Before they can even make their own experiences, they basically acquire their mothers' experiences. Most importantly, these maternally transmitted memories are long-lived, whereas other types of infant learning, if not repeated, rapidly perish."

In the new study, Dr. Debiec and colleagues exposed non-pregnant rats to an unpleasant electric shock whenever they smelled peppermint. After the rats became pregnant and gave birth, the researchers again exposed the mother rats to peppermint smell, this time in the presence of their newborns. The mothers exhibited physical symptoms of fear.

When the newborns grew to maturity, the researchers again exposed them to the odor of peppermint. Though these rats had never been shocked when exposed to this smell, their levels of stress hormones rose in its presence, indicating fear. This reaction is particularly notable given that their prior exposure to peppermint had been when they were too young to see or otherwise observe their environments.

"During the early days of an infant rat's life," Dr. Debiec said, "they are immune to learning information about environmental dangers. But if their mother is the source of threat information, we have shown they can learn from her and produce lasting memories."

Learning by smell
The researchers later repeated the experiment, but this time, rather than exposing the mother to peppermint in the presence of her newborns, they did so while she was alone. They then piped in the air from the mother's enclosure to the newborns. When these rats were re-exposed to peppermint smell as adolescents, their stress hormone levels still went up -- suggesting that the information had been transmitted by smell.

Using brain imaging and data on genetic activity and stress hormone levels, the researchers determined that the region of the brain known as the amygdala might be responsible for teaching the newborns to adopt their mothers' fears. This was confirmed when the researchers gave the newborn rats a drug to block amygdala activity while exposing them to the smell of their mothers reacting to peppermint. These rats later showed no reaction to peppermint odor.

According to the researchers, the amygdala plays a key role in detecting and responding to threats.

Although the study was conducted in rats, the researchers believe that a similar mechanism may explain how parents (including fathers, if they are regular caretakers) transmit some fear to their children, such as fear of the dentist or extreme shyness. Other studies have already shown that babies can be calmed by the scent of their mother; perhaps they can absorb her fear, as well.

Humans might also signal fear to their offspring in other ways, such as through changes in facial expression, tone of voice or body language.