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Scientists believe they may have cracked the secret of why some people are always in a bad mood.
They have found that people who are regularly grumpy have an overactive area in their brains.
The discovery could eventually lead to new treatments for depression.
Scans of healthy people's brains show that those who report having been in a bad mood recently have increased activity in a region of the brain called the ventromedial prefrontal cortex.
It lies an inch or two behind the right eye in right-handed people and has been linked with emotions in other studies.
Chicken or egg?
Psychologist David Zald, of Vanderbilt University in Nashville, who led the study, said it is not yet clear whether the brain activity causes the bad mood, or vice-versa.
However, he said: "Such a connection does make sense because animal studies show that this region of the brain controls heart rate, breathing, stomach acidity levels, sweating and similar autonomous functions that have a close connection to mood."
The researchers did two studies of a total of 89 people. They used a brain imaging technique called positron emission tomography (PET) to measure brain activity.
Their volunteers had the PET scans taken while sitting quietly. They also filled out standard questionnaires designed to measure mood as objectively as possible.
They assessed their immediate mood and also how they had felt over the past month.
Those who reported a lot of irritability, anxiety or anger had extra blood flow in the ventromedial prefrontal cortex.
People with mood disorders such as anxiety, or those with depression, show clear differences in brain function.
But Dr Zald's team said their finding clearly showed differences could also be seen in the brains of normal, healthy people.
Dr Zald said: "With increased knowledge of the relationship between brain function and mood, we should be able to find more effective ways to treat the millions who suffer from clinical depression each year."
Dr Frederick Wilson, a neuropsychologist at Thompson House Hospital, Lisburn, Northern Ireland, told BBC News Online that there was a long history of people with frontal lobe lesions have problems with their mood.
He said: "If this finding could be reproduced in a patient population with aggitated depression then it would be very interesting.
"However, such a finding in a non-patient population does not necessarily have implications for a patient population."
The research is published in the Proceedings of the National Academy of Sciences.
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