How grumpy people feel can be influenced not only by the state of mind of those closest to us, but also by friends of friends that we have never met like ripples from "pebbles thrown into a pond" , claim scientists.
Professor James Fowler of the University of California, said each 'unhappy connection' decreased the chance of a person being happy by seven per cent.
However, the effect works both ways with each 'happy connection' increasing the chance of a person's happiness by nine per cent.
The team led by Dr Nicholas Christakis, a sociologist at Harvard Medical School, and including Professor Fowler, made their conclusions after analysing 53,228 social connections between 5,124 individuals over time.
Dr Christakis said: "If you drop one pebble in a pond, it will create ripples out from the pebble."
He added: "Most people will not be surprised that people with more friends are happier, but what really matters is whether those friends are happy."
Moods - good and bad - tended to travel more freely in same-sex groups, they report in New Scientist magazine.
They found a happy friend increased the odds of someone being happy by 15 per cent - but that a friend of a friend boosted the chance by about 10 per cent, and a friend of a friend of a friend by about six per cent.
That was triple the two per cent chance of being happier caused by a £3,500 pay rise.
Despite physical proximity, the mood of a cohabiting partner was less than 10 per cent likely to have an effect.
Behaviour including drinking, smoking and even obesity can spread in a similar manner, the team claim.
Last year his team found a person's risk of gaining weight increased significantly when friends gained weight. If their friends gained weight a similar but lesser effect was evident on the individual.
He said: "Obesity appears to spread through social ties."
Sceptics may question how a person one has never met can have a direct influence on mood, and ask how the effects of individual friendships can be teased out of a mass of social connections.Original here