Children inherit a lot of things from their parents. The color of the hair and eyes, height and intelligence are just some of the traits that people are known to inherit from their parents. Clinical evidence also suggests that fear is something that can be passed on across generations albeit scientists do not clearly understand how this occurs.

An experiment with rats, however, suggests that a mother's fear can be transmitted to her children and smell has something to do with how this happens. In the new study published in the Proceedings of the National Academy of Sciences on July 28, Jacek Debieca and Regina Marie Sullivana, from the Department of Child and Adolescent Psychiatry of the New York University School of Medicine, taught female rats to associate the smell of peppermint with electric shocks before they get pregnant and exposed them to the minty smell again but without the electric shock after they gave birth to provoke fear.

The researchers then exposed the pups of these mother rats to the same smell with and without their mothers and observed that the peppermint smell appeared to have made the pups anxious as well.

 "During the early days of an infant rat's life, 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," Debiec said.

Debiec and Sullivana likewise observed that the pups have learned to fear the smell of the peppermint even without the presence of their mother. Just the piped-in scent of the mother rats reacting to the feared minty odor appeared to be enough to make the pups fear the same thing. When the researchers gave the pups a substance that blocked the activity in the amygdala, they observed that the baby rats did not learn the fear of peppermint smell from their mothers.  

 "Maternal presence was not needed for fear transmission, because an elevation of pups' corticosterone induced by the odor of the frightened mother along with a novel peppermint odor was sufficient to produce pups' subsequent aversion to that odor," the researchers wrote.

The odor that mother rats release when they feel distressed apparently taught babies what to fear during the first days of their lives. Debiec and Sullivana have also noted that some mother rats attempted to stop the minty smell from coming through, a behavior that the researchers have found interesting and wanted to study further.