A gene which has been linked to aggression in men appears to make women happy, according to a US study.
Researchers from the University of South Florida, the US National Institutes of Health, Columbia University and the New York State Psychiatric Institute found the low-expression form of the gene monoamine oxidase A (MAOA) is associated with higher self-reported happiness in women.
However no such association was found with men.
The study's lead author Dr Henian Chen, a researcher at the University of South Florida College of Public Health, was surprised by the findings.
"This is the first happiness gene for women," Dr Chen said.
"I was surprised by the result, because low expression of MAOA has been related to some negative outcomes like alcoholism, aggressiveness and antisocial behaviour.
"It's even called the 'warrior gene' by some scientists, but, at least for women, our study points to a brighter side of this gene."
Researchers analysed data from 345 people - 193 women and 152 men - participating in Children in the Community, a longitudinal mental health study.
Participant's DNA was analysed for the MAOA gene variation and their self-reported happiness was scored by on a scale.
The MAOA gene regulates the activity of an enzyme that breaks down serotonin, 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.
Compared to women with no copies of the low-expression version of the MAOA gene, females with one copy scored higher on the happiness scale and those with two copies increased their score even more.
While a substantial number of men carried a copy of the "happy" version of the MAOA gene, they reported no more happiness than those without it.
The researchers suspect the testosterone hormone may cancel out the positive effects of the gene on the happiness of men.
The potential benefit of MAOA in boys could wane as testosterone levels rise with puberty, Dr Chen said.
"Maybe men are happier before adolescence because their testosterone levels are lower," he said.
While women experience higher rates of mood and anxiety disorders than men, they tend to report greater overall life happiness than their male counterparts. The reason for this is not clear, Dr Chen said.
"This new finding may help us to explain the gender difference and provide more insight into the link between specific genes and human happiness."
Dr Chen said more research is required to identify which specific genes influence resilience and subjective wellbeing, as studies of twins estimate genetic factors account for 35 to 50 per cent of the variance in human happiness.
While happiness is not determined by a single gene, there is likely to be a set of genes that, coupled with life experiences, shape our individual happiness levels, Dr Chen said.
"I think the time is right for more genetic studies that focus on well-being and happiness."
The findings have been published online in the journal Progress in Neuro-Psychopharmacology and Biological Psychiatry.