Vulnerability to anxiety may be down to the size of a brain structure involved in fearful memories, say US scientists.
A small vmPFC may mean you are prone to anxiety
People with a thicker ventromedial prefrontal cortex were better able to cope with stressful experiences.
The findings may help explain why some people develop post-traumatic stress disorder (PTSD) while others bounce back after adversity, say the authors.
The Massachusetts General Hospital study appears in Proceedings of the National Academy of Science.
While it is normal to experience physical and psychological symptoms after an extremely stressful event, such as the recent London terrorist attacks, some people will continue to be consumed by overwhelming fear and may develop PTSD.
A person with PTSD may experience unwanted flashbacks, poor sleep and depression, and avoid certain situations that could trigger memories of the event.
Studies in animals suggest that the ventromedial prefrontal cortex (vmPFC) is involved with helping the brain forget fearful events.
Also, studies have shown that people with PTSD have unusually inactive vmPFCs, again suggesting that this brain region is important in anxiety.
In the current study, Dr Mohammed Milad and colleagues scanned the brains of 14 volunteers.
The volunteers were also exposed to a series of experiments, involving harmless but uncomfortable electric shocks, which were designed to cause anxiety.
The volunteers who had the least anxiety responses, gauged by how sweaty their palms were during the tests, tended to have thicker vmPFCs and vice versa.
Dr Milad said: "These results suggest that a bigger vmPFC may be protective against anxiety disorders or that a smaller one may be a predisposing factor."
However, he said they did not yet know how that might work.
His colleague Dr Scott Rauch said the next step was to look at genetic factors and factors in the environment that might explain the brain differences.
In the future, it might be possible to measure a person's vmPFC to predict whether they are more prone to anxiety disorders such as PTSD.
Dr Cosmo Hallstrom, consultant psychiatrist in London, said: "We know that some people are more vulnerable to stress and anxiety and it is nice to have a biological correlate of that.
"Certainly, that part of the brain is associated with a whole manner of psychiatric vulnerabilities.
"It is not surprising that anxiety disorders may also have part of their underlying vulnerability in that area."
He said the important thing to recognise was that PTSD is treatable and should be managed as early as possible.