Happiness gene discovered, but works only in women, states study.
The findings could help explain why women are often happier than men, the predominantly female team of US researchers said.
The study focused on a gene called MAOA that affects the levels of feel-good chemicals in the brain.
Almost 350 men and women were asked how happy they were and gave a saliva sample that was tested for their DNA.
The MAOA gene comes in two versions, a highly active and a less active type.
Women with the less active type were happier than others, the researchers found - and those with two copies of the "happy" type of MAOA had the sunniest dispositions of all.
Some 59 percent of the women studied had one copy and a lucky 17 percent had two - leaving only around one in four women lacking the happiness gene.
Many of the men in the study carried the gene but were no happier than others.
University of South Florida researcher Dr Henian Chen said this could be because its effects are cancelled out by testosterone - but boys may benefit.
"Maybe men are happier before adolescence because their testosterone level is lower," the Daily Mail quoted him as saying.
The link was true even when other factors affecting wellbeing - from age to income - were taken into account.
But there was a downside - the happy version of the gene has been linked to alcoholism and anti-social behaviour.
"You may be more likely to get into trouble but still be happier than the next person," Dr Chen said.
The findings are published in the journal Progress in Neuro-Psychopharmacology and Biological Psychiatry.