So there is no point in telling the Victor Meldrews and Albert Steptoes of the world to cheer up - their brains are simply designed to be more grumpy than others.

Psychologist Dr David Zald of Vanderbilt University in Nashville has identified a tiny part of the brain which he believes governs people's tendency to have regular bouts of irritability, anxiety or anger. The more active that part of the brain, the more likely someone is to suffer bad moods.

'It looks like it is this part of the brain's activity that regulates people's mood. It is also a part of the brain that controls sweating, stomach acidity and heart rate and other physical feelings associated with stress and bad moods,' said Zald.

The culprit is a postage stamp-sized bit of the brain called the ventromedial prefrontal cortex, which lies an inch or two behind the right eye. Zald conducted studies on 89 people, scanning their brains using a technique called positron emission tomography, or PET. They were then asked to complete a detailed survey of their moods over the last month.

Those who experienced a lot of bad moods were also revealed to have a lot of extra activity in the ventromedial prefrontal cortex.

Zald's research is backed up by studies on people who have suffered damage in that area of the brain. These people often lose the ability to feel anxiety or stress. 'They could literally think to themselves: "Should I bet that $10,000 on a roll of the dice?" and not feel anything in the pit of their stomach. They just don't feel anxiety, which is linked to bad moods,' said Zald.