John Cacappio of the University of Chicago and his colleagues reckon that loneliness can spread through society like an infection.

Their study, published in this month's issue of the Journal of Personality and Social Psychology, draws on a large group of people living in a town in Massachusetts that had already been assigned as part of a heart health study.

This group of around 5,000 were given questionnaires to assess their loneliness every four years or so, enabling the researcher to track any "spread" in this gloomy emotion.

Cacappio reckons that if you're around lonely people, you're more likely to "catch" their loneliness. For example, for every day your next door neighbour is lonely, you're likely to experience around 5 hours of loneliness.

The authors also come up with some interesting figures on the spread of loneliness.

Perhaps it's obvious that having friends helps you avoid being lonely, but Cacioppo has put a number on the value of friendship: each friend you have apparently saves you from 2 days of loneliness a year.

It's not only your friends and neighbours who can put you at risk, say the authors. In their paper, they say that you can even "catch" loneliness from your friend's friend's friend, through 3 degrees of separation.

And the reason we aren't all lonely and miserable now is that we tend to drive lonely individuals out of society so they don't "infect" the rest of us, says Cacappio. Lonely people tend to cluster at the edges of their social groups as a result, shown graphically here.

Chris Segrin, a behavioural scientist at the University of Arizona who was not involved in the study, agrees that lonely people were more likely to spend time together, and worsen each others' moods.

"It's called emotional contingent, where I catch your mental state," he told ABC News. "If I'm hanging out with you and you're bringing me down, maybe what I need to do is think about a new circle of friends."

Cacappio reckons that it is important to recognise loneliness before it spreads, because of the effects that the low feeling can have on health.

Being lonely is thought to weaken your immune system, and Cacappio's previous research suggests that it can have the same effect on your blood pressure as smoking.

Not everyone is excited about the implications of the study, though. Jason Fletcher, assistant professor of public health at Yale University, flagged concerns about the limitations of the study in an email to the Washington Post.

Last year, Fletcher and Ethan Cohen-Cole warned fellow researchers that social network effects can be found very easily when environmental factors aren't fully acknowledged.

In a study published in the British Medical Journal, the pair found that acne, headaches and even height seemed to have "network effects", spreading through social networks. These effects were insignificant, however, when environmental factors were accounted for.

Cacappio's team all but dismiss these factors in their study. They think that the nature of a friendship makes a difference to the spread of loneliness, and that this rules out an effect of any shared lonely environment.

http://www.newscientist.com/blogs/shortsharpscience/2009/12/the-loneliness-of-three-degree.html