Mothers may pass on fears via smell
Mothers teach newborn babies what to be scared of by literally letting them smell their fear, new research suggests.
In a study of rats, scientists showed how a mother's alarm scent primed her days-old offspring to develop a peppermint phobia.
They believe the same "smell of fear" mechanism may be at work in humans.
For generations experts have been puzzled by the degree to which a mother's traumatic experiences can affect her children.
Some children of Holocaust victims, for instance, react as if they had witnessed the same horrors as their mothers.
Their fear, manifested in the form of nightmares, avoidance behaviour and even flashbacks, appears too deeply ingrained to have been learned simply by hearing stories.
US study leader Dr Jacek Debiec, from the University of Michigan, said: "Our research demonstrates that infants can learn from maternal expression of fear, very early in life.
"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."
Polish-born Dr Debiec found evidence of human babies "inheriting" fear from their parents while working with the grown children of Jews who survived the Nazi death camps.
He hopes the research will lead to ways of blocking the transmission of irrational or harmful fear responses from mothers to their children.
The research, published in the journal Proceedings of the National Academy of Sciences, pinpoints a specific part of the brain - the lateral amygdala - where smell-transmitted fear takes root early in life.
The scientists studied mother rats who were taught to associate the smell of peppermint with a mild but unpleasant electric foot shock.
After giving birth, the rats were again exposed to the minty smell in the presence of their newborn pups.
A frightened mother's odour transmitted the same fear to her offspring, whose stress hormone levels rose, the researchers found.
The newly acquired fear was also long lived, lasting through early adolescence.
A further experiment showed that smell alone was able to imprint fear into a baby rat, without its mother having to be present. Just the piped-in scent of a mother reacting to peppermint was enough to make a rat pup frightened of the same thing.
By using a drug to block activity of the amygdala, the scientists were able to prevent baby rats learning to fear peppermint from their mothers' smell.
While it is not yet known if the same smell-based fear signalling occurs in people, human mothers' scent has been shown to have a calming effect on babies.
The scientists wrote: "Despite clinical evidence that specific fear is transmitted across generations, we have little understanding of mechanisms.
"Here we model social transmission of mother-to-infant fear in rodents. We show that maternal fear responses to a conditioned fear odour are sufficient to induce robust fear learning throughout infancy, with robust retention.
"Elucidating the mechanisms of this transmission may inform the development of novel therapeutic and preventive approaches."