Loneliness is like a disease — and what's worse, it's contagious.
It may sound counterintuitive, but loneliness can spread from one person to another, according to research released this week that underscores the power of one person's emotions to affect even people they don't know.
The federally funded analysis of data collected from more than 4,000 people over 10 years found that lonely people increase the chances that someone they know will start to feel alone, and that the solitary feeling can spread.
"Loneliness is not just the property of an individual. It can be transmitted across people — even people you don't have direct contact with," said John T. Cacioppo, a University of Chicago psychologist who led the study published in the December issue of the Journal of Personality and Social Psychology.
Moreover, people who become lonely eventually move to the periphery of their social networks, becoming increasingly isolated, which can exacerbate their loneliness and affect social connectedness, researchers found.
"No man is an island," said Nicholas A. Christakis, a professor of medicine and medical sociology at Harvard Medical School who worked on the research. "Something so personal as a person's emotions can have a collective existence and affect the vast fabric of humanity."
Loneliness has been linked to medical problems, including depression, sleep problems and generally poorer physical health. Identifying some of the causes could help reduce the emotion and improve health, experts said.
Although the study did not examine how loneliness spreads, Cacioppo said other research has provided clues.
"Let's say for whatever reason . . . you get lonely. You then interact with other people in a more negative fashion. That puts them in a negative mood and makes them more likely to interact with other people in a negative fashion and they minimize their social ties and become lonely," Cacioppo said.
For the study, Cacioppo teamed up with Christakis and James H. Fowler, an associate professor of political science at the University of California at San Diego, who have published a series of papers and the book Connected, based on data originally collected by the Framingham Heart Study, a long-running project that has explored a host of health issues.
The new analysis, involving 4,793 people who were interviewed every two years between 1991 and 2001, showed that a friend of a lonely person was 52 percent more likely to develop feelings of loneliness by the time of the next interview. A friend of that person was 25 percent more likely, and a friend of a friend of a friend was 15 percent more likely.
Loneliness spread more easily among women than men, perhaps because women were more likely to articulate emotions, Cacioppo said.
The findings underscore the importance of social networks, several experts said.
"For years, physicians and researchers thought about individuals as isolated creatures," said Stanley Wasserman, who studies social networks at Indiana University. "We now know that the people you surround yourself with can have a tremendous impact on your well-being."