New research from experts in neuroscience and social science may give us a clue as to why. Although we tend to think of it as a self-contained emotional state — a condition that affects people individually, either by circumstance or by dint of an antisocial personality — researchers now argue that loneliness is much farther reaching than that. John Cacioppo, a psychologist at the University of Chicago, believes it is a social phenomenon that exists within a society and can spread through it, from person to person, like a disease. And while everyone feels lonely once in a while, for some it becomes a persistent condition, one that has been associated with more serious psychological ills like depression, sleep dysfunction, high blood pressure and even an increased risk of dementia in older age.

For his latest study, published in the Journal of Personality and Social Psychology, Cacioppo partnered with leading social-network scientists Dr. Nicholas Christakis of Harvard University and James Fowler of the University of California, San Diego, the team best known for their series of studies showing that emotional states and behaviors — including happiness, obesity and quitting smoking — can propagate like a wave throughout a network of people. To examine whether the contagion effect existed with loneliness, the researchers used the same data set that Christakis and Fowler had mined for their earlier studies — the Framingham Heart Study, an ongoing trial originally begun in 1948 to identify risk factors for cardiovascular disease. Thanks to the meticulous way the trial was initially set up, with investigators noting the close family members and friends of each participant to ensure follow-up over the years, Cacioppo, Christakis and Fowler now had access to a rich social network for each volunteer in their study — from family members and friends to colleagues and neighbors.

Cacioppo and his team focused on the children of the original Framingham cohort, which included more than 5,200 middle-aged men and women. Starting in 1983, more than 4,500 volunteers were asked to fill out three questionnaires, spaced two years apart, about how many days in the previous week they had felt lonely. Because most of the participants' friends and family members were also part of the Framingham study, the scientists could track, over time, whether one person's report of loneliness had any impact on the feelings of isolation in other members in his or her social network. Researchers were thus able to rule out the possibility that lonely people simply congregated with other lonely people, or that a shared environmental event, such as a fatal fire in the neighborhood, could have triggered mass feelings of loneliness.

The results were illuminating: If one person reported feeling lonely at one evaluation, his closest connections (either family or close friends) were 52% more likely to also report feeling lonely two years later. The effect was strongest among those in close relationships, waning as the connections became more distant, but remained significant up to three degrees of separation — in other words, one lonely person could influence whether his friend's friend's friend felt lonely. "Loneliness has been conceived in the past as depression, introversion, shyness or poor social skills," says Cacioppo. "Those turn out not to be right. Research we and others have done suggests that it really is a fundamental human motivational state very much like hunger, thirst or pain."

In other words, loneliness is not so much a symptom of being companionless, as it is a driving force behind social isolation. Rather than simply reflecting the emotional state of one person, Cacioppo says, loneliness is more like an indicator of the social health of our species on the whole — a temperature reading, if you will, of how well or not so well-integrated we are as a population.

That's an important measure, he says, because we are, by nature, a social species; we feed off of our interactions with each other and thrive when we are inspired, challenged, and supported by one another. While occasional feelings of isolation are perfectly natural, and normal, the new study suggests that loneliness can begin to fester in a society like a cancer if it is allowed to transmit unchecked from one person to another.

But how does a person "catch" loneliness? Based on the new data, Cacioppo theorizes that it is passed on through feelings of mistrust and negativity. "People who feel lonely view the social world as more threatening," he says. "They may not be aware they are doing it, but lonely individuals think negatively about other people. So if you are my friend, and I started to treat you negatively, then over time, we would stop being friends. But in the meantime, our interactions caused you to treat other people less positively, so you're likely to lose friends, and they in turn are likely to lose friends. That appears to be the means of transmission for loneliness." People may be spreading their negative feelings simply by frowning or making other unpleasant facial expressions, throwing out hurtful remarks, or even adopting uninviting body postures.

Over time, lonely people find themselves banished to the periphery of their social networks; as they lose friends and connections, they are pushed to the fringes, where they are only marginally connected to the community. Viewed that way, say experts, the loneliness factor in a neighborhood or an apartment complex or a workplace may be an indication of how cohesive, and therefore, mentally healthy, that population is. "Loneliness can be a signal for when that social connection is fraying," says Cacioppo.

If these results hold up, then, treating loneliness should involve more than individual therapy for patients. It requires addressing larger, society-based issues. "People are not going to realize that there is almost a wave of loneliness that is being propagated by people two or three connections removed from them," says Dr. Richard Suzman, director of the division of behavioral and social research at the National Institute on Aging, which funded the new study. "This does suggest that one has got to look at both the network and individual simultaneously when you try to repair what seems to be a cascading, spiraling descent in which loneliness gets increasingly paired with isolation."

That strategy may mean looking at things such as community design or social support networks that allow some populations to keep all their members hovering near the center of their networks, rather than drifting to the edges. It's not necessarily the number of connections people have that matters, but the quality of them. Communities that encourage regular interaction among its members, either through regular gatherings or mutually beneficial projects that require everyone's input, for example, are more likely to foster stronger, more meaningful connections than those that don't encourage social investment. "Ultimately what we hope to do is not only intervene at the individual level, but also at the city planner and development level as well," says Cacioppo.