Loneliness is contagious and expressing it can make us even more isolated, according to new research from Harvard, the University of Chicago and the University of California-San Diego.
Looking at a longitudinal study of more than 12,000 people, the researchers found that lonely respondents "infected" remaining friends with their loneliness before the relationships crumbled, perpetuating a cycle of isolation.
Loneliness spreads because even as lonely people seek social connection, their "caustic" behaviour often frays relations down the line, says John Cacioppo, a University of Chicago psychologist and one of the study authors.
Loneliness, he says, is a "sensitivity to social rejection" that makes our brains more alert to social threats - and subsequently makes us harder to be around.
Lonely people tend to be shyer, less trusting and more socially awkward, anxious and hostile, wrote the authors, whose findings were published in the December issue of the Journal of Personality and Social Psychology.
The researchers found that next-door neighbours and friends were more likely to make each other lonely than siblings or spouses, and that women were more likely to "catch" and spread loneliness than men.
They say their findings have particular implications for older people. Previous research has shown that loneliness can play a serious role in their health, including cardiovascular risks, the progression of Alzheimer's, obesity, alcoholism and depression.
"Social species do not fare well when forced to live solitary lives," the authors write, comparing loneliness to hunger, thirst and pain.
The researchers tracked the "topography" of loneliness in Framingham, a town in Massachusetts whose residents had been tested extensively since 1948 for a study of cardiovascular risk factors.
The townspeople filled out a battery of surveys on depression and loneliness and also listed the names of all their family members and friends on tracking forms. Many of these acquaintances lived in Framingham and underwent the same study, giving the authors of the loneliness research a unique glimpse into the town's social networks.
The researchers had already used the Framingham sample as the basis for a number of inquiries into the process of "contagion," tracing how obesity, smoking and happiness can spread through social networks and communities.
As with the other studies, the researchers found that loneliness spreads through three degrees of separation.
"Participants are 52 per cent more likely to be lonely if a person to whom they are directly connected (at one degree of separation) is lonely," the authors write.
At two degrees of separation, they were 25 per cent more likely to feel lonely. At three degrees it was 15 per cent and at four degrees the effect disappeared. This pattern - what the authors term the "three degrees of influence rule of social contagion" - also appeared in the obesity, smoking and happiness studies.
Lonely people in Framingham often cut the few ties they had left, having transmitted "the same feeling of loneliness to their remaining friends, starting the cycle anew."
The researchers found that lonely spouses rubbed off much less than lonely friends. Lonely siblings appeared to have no effect on each other, suggesting that "loneliness in older adults is about the relationships people choose, rather than the relationships they inherit," the authors wrote.
The study suggests loneliness is not a "quintessential individualistic experience," but a complex group dynamic, one that remains stigmatized.
"We have this notion that it's personal weakness," Dr. Cacioppo says.
He argues that today's culture is particularly vulnerable to loneliness because we are postponing family, divorcing more often and living longer.
The average person spends 80 per cent of their waking hours in the company of others, and prefers that to time alone, the authors write, citing earlier studies.
The researchers are now looking for predictors of social isolation in south Chicago neighbourhoods, and hope their work will eventually trickle down to city policy.
"[The research]implies cities, communities, neighbourhoods, buildings and roads can be constructed in a way that either promotes social cohesion and a feeling of connection or places that at high risk," Dr. Cacioppo said.
Other members of the study team were James Fowler, associate professor of political science at the University of California-San Diego, and Nicholas Christakis, professor of medical sociology at Harvard Medical School.