Scientists have discovered a gene that appears to control whether fear reactions to impending danger are appropriate or not.
Mice lacking the gene were less fearful
Mice lacking the gene stathmin appeared fearless in conditions that should instinctively inspire fear.
The gene is expressed in particularly high levels in a part of the brain, called the amygdala, known to be important in human fear.
The US team told Cell their findings could shed light on anxiety disorders.
The same researchers from the Howard Hughes Medical Institute identified a similar gene a few years ago, called GPR that appeared to be important in the process of "learned fear".
This is where animals, including humans, learn over time that something is a threat or danger, as opposed to the instinctive fear which animals are born with.
GPR appeared to block the action of "circuitry" in the amygdala of the brain which learns fear.
Conversely, the newer gene discovered, stathmin, appears to help this circuitry.
Mice bred to lack stathmin showed abnormally low levels of anxiety in situations that would normally make a mouse very afraid, such as being in large open spaces - innate fear.
They also reacted less to learned fear.
In the study, this was a neutral tone that was played while the animals were delivered a mild electric shock.
The mice showed a decreased memory for the fearful situations and had difficulty recognising dangerous environments.
Their memory for things other than fear was not impaired, however.
Co-researcher Dr Gleb Shumyatsky, from Rutgers University Piscataway, New Jersey, said these mice could be used to study human phobias and anxiety-related disorders such as post-traumatic stress disorder.
"These animal models could be used to develop new anti-anxiety agents," he added.
He said that taken together, their work on genes and anxiety supported clinical data indicating that anxiety is a spectrum of disorders.
He said it was likely that each disorder would have a "unique molecular signature" and therefore require individually tailored drugs for treatment.
Professor Alexander Gardner, a clinical psychologist in Glasgow and member of the British Psychological Association, said: "There is already evidence that the amygdala is involved in fear.
"This is very interesting research indeed."
He said it was important for animals and humans to recognise and respond to fearful situations for survival.
However, he questioned whether fear could be truly innate.
For example, he argued that man was not naturally afraid of fire - man used it to be able to exist in colder climates - but we learn that it can be dangerous and therefore do fear it.
He suggested it might be that certain genes laid down a map in the brain for further acquisition of fear.