For reasons that scientists have not conclusively determined, women are happier than men. And now, researchers think that they may have pinpointed one of the reasons for that. They have found a gene in women that predicts the level of happiness in women.

Though women suffer from anxiety and depression more than men do, women also report levels of happiness higher than men. Henian Chen from the University of South Florida and her colleagues from the National Institutes of Health, Columbia University, and the New York State Psychiatric Institute searched for a biological reason for that discrepancy. They traced it to the low-activity form of monoamine oxidase A (MAOA).

The findings surprised the researchers, because that same gene has been linked to alcoholism, aggression and generally antisocial behavior.

The investigators analyzed a group of 345 people - 193 women and 152 men. When they controlled for age, education level, and income, among other factors, they found that women with one copy of MAOA reported a larger amount of happiness than did women with no copies. Women with two copies of the gene reported an even larger boost.

The statement released by the University of South Florida explains, "The MAOA gene regulates the activity of an enzyme that breaks down serontin, dopamine and other neurotransmitters in the brain -- the same "feel-good" chemicals targeted by many antidepressants. The low-expression version of the MAOA gene promotes higher levels of monoamine, which allows larger amounts of these neurotransmitters to stay in the brain and boost mood."

Interestingly, the gene did not hold the same correlation for men, who reported the same amount of happiness no matter if they had zero, one or two copies.

Researchers suspect that testosterone may play a role in the difference. Women have less of it than men and the researchers think that the hormone may cancel out the positive effects of the gene. They also think that the effects of the gene may wane with puberty in men, when testosterone levels increase.

Of course, researchers say that no gene can define anyone's happiness. A person's outlook on life is often shaped to a large degree by their circumstances and previous life events.

But researchers add that studies of twins indicate that genetic factors provide 35 to 50 percent of the variance in human happiness.

The study was published in Progress in Neuro-Psychopharmacology and Biological Psychiatry.