Around one in four Britons have a gene scientists believe is linked to higher levels of happiness, according to a study.
Researchers analysed 14 years of World Values Surveys (WVS) and compared them with a genetic database showing the frequency of a certain DNA code in the FAAH gene.
They found those with the sequence were more likely to say they were "very happy".
An estimated 23% of Britons have the code - the A allele - in the FAAH gene, the academics from Varna University of Management in Bulgaria and Hong Kong Polytechnic University said.
People from Mexico and Nigeria had the highest prevalence of the genetic code and were the happiest, they added.
The A allele in the FAAH gene is linked to increased levels of a fatty acid which is said to decrease anxiety and it is also more prevalent in certain ethnic groups, including Amerindians.
The study's authors, Michael Minkov and Michael Bond, said their results showed that happiness is not necessarily linked to a nation's economic growth, as has been suggested in other studies.
They wrote: "This is the first study showing that national differences in happiness, defined as the hedonic component of subjective well-being or positive effect, have a genetic component.
"Nations with the highest prevalence of the A allele in rs324420 of the FAAH gene have the highest percentages of very happy people, and this association is quite strong.
"Vice versa, nations with the lowest prevalence of that allele have the lowest percentages of very happy people.
"The lowest prevalence of the A allele is found in some Arab and East Asian nations, most of which have low happiness scores.
"Differences in happiness between Northern and Central or South Europeans also seem attributable to the genetic differences between them, since Northern Europeans have a much higher prevalence of the A allele."
The academics admitted their findings were based on estimates and further studies would be needed to be done to confirm the genetic link.