Happiness is in your genes it seems, according to research that shows the more people in a country who have a particularly gene, the happier the nation will be.

The DNA in question, the FAAH gene, makes a protein that affects feelings of pleasure and pain. People with a particular version of it tend to be cheerier souls.

However, wealth and health were found to have little effect on happiness. The researchers said the find could help explain why some of the world's poorest nations are also the happiest.

The team from Bulgaria and Hong Kong looked at whether there was a link between levels of the FAAH gene in a population and number of people who said they were 'very happy' in global study of life satisfaction.

Sweden – one of the happiest countries in Europe and in the world – also had lots of happy DNA. Some 26.3 percent of Swedes have the happiness gene, compared to 23 percent of Britons, 21 percent of the French and 20 percent of Germans.

The happiness gene is even rarer in southern Europe, where it is found in 18 per cent of Greeks and just 12 percent of Italians.

Further afield, Ghana, Nigeria, Mexico and Columbia all came out near the top in the happiness league – and sported high rates of the gene.

In contrast, the peoples of Iraq, Jordan, China were among the least likely to rate themselves as 'very happy' and also had the lowest levels of the gene.

But the gene and an optimistic outlook didn't always go hand in hand.

For instance, Russians and Estonians score very low on happiness, despite having the 'right' DNA, theJournal of Happiness Studiesreports.

Climatic differences were also found to be significantly associated with national differences in happiness.

Study co-author Michael Minkov, of the Varna University of Management, said: 'We cannot fail to notice the high occurrence of the gene in equatorial and tropical environments in the Americas and Africa - and the lower occurrence of [it] around the Mediterranean Sea than in Northern Europe.

A spokesman for the journal's publisher, Springer, said: 'Genetics is not the only determinant of happiness.

'The economic and political difficulties continuously experienced by Eastern European nations contribute to the very low happiness scores of Russians and Estonians.'

Researcher Professor Michael Minkov, of Sofia's Varna University, said that nations not blessed with the right DNA aren't necessarily destined to be miserable— happiness can still raise and fall for other reasons.