Women have lower levels of a crucial enzyme

They have shown that women have lower levels of a brain chemical that controls anxiety.

Having less of the chemical, an enzyme called COMT, appears to make a person more anxious and highly strung.

And women who have a particular version of the gene that makes the enzyme are most likely to worry most of all.

Scientists at the National Institute on Alcohol Abuse and Alcoholism (NIAAA) combined DNA analysis, recordings of brain activity and psychological tests.

They found that women with one particular variation of the gene - called Val 158Met - scored high in tests that measured their anxiety levels.

Scans of electrical activity in the brain also showed signs that they had an anxious temperament.

Everybody inherits two copies of the gene - one from each parent.

Women who inherited two copies of the Val 158Met gene were the greatest worriers of all.

However, men who had the same genetic make-up did not appear to be extra anxious.

Pain

Researcher Dr Mary-Anne Enoch said the variant gene was linked to a threefold to fourfold decrease in COMT enzyme levels.

"Our study suggests that women with this genotype may be more vulnerable to anxiety because their COMT levels fall below a minimum threshold."

The study also highlighted the fact that all women - not just those with the gene variant - were bigger worriers than men.

Previous studies have shown that women generally have lower COMT levels.

A recent study at the same laboratory found that the variant gene was also associated with a higher level of brain response to pain and stress.

COMT levels have also been shown to influence mental awareness, alcoholism and schizophrenia.

Professor Stephen Bloom, a stress expert at Imperial College London, told BBC News Online that more research would be needed to show a definite link between COMT and human behaviour.

"The human brain is immensely complicated, and has all sorts of compensating changes which regulate behaviour," he said.

"The conclusions here are out of proportion to the results, and based on very little."

The findings were published in the journal Psychiatric Genetics.