The lonely brain is different from the non-lonely brain, says John Cacioppo, director of the Center for Cognitive and Social Neuroscience at the University of Chicago and one of the nation's leading experts on the neurobiology of loneliness. In people like me, who for various reasons are primed to define ourselves as lonely — more on those reasons later — the brain switches easily into self-preservation mode when we're feeling loneliest, quick to see social danger even when it isn't there.

In one MRI study, for instance, Cacioppo put subjects into a scanner and showed them negative images that had either a social or a non-social context. (A negative social image might be a picture of two men arguing; a negative non-social image might be of a shark.) Subjects who had been identified as lonely paid more attention to the negative social images. "The lonelier the brain," Cacioppo said at a TEDx talk last year, "the more visual cortical activity is devoted to that negative social image." Lonely people also showed less activation, when looking at negative social images, of the temporal parietal junction, the brain region involved in taking another person's point of view.

In another study, Cacioppo brought lonely and non-lonely young adults into a sleep lab. The lonely subjects, he found, had more disordered, less restorative sleeping, with more micro-awakenings during the night, almost as though they were remaining vigilant for social rejection — or for threats of any kind — even as they slept. As a result they didn't feel refreshed after sleep, and tended to get drowsy during the day.

A few years ago, Cacioppo and his colleague, Louise Hawkley, summarized a collection of psychological studies linking loneliness to a variety of mental health problems: increased negativity, depressive thinking, heightened sensitivity to social threats, and trouble with impulse control. As a result, they wrote, lonely people are impaired in their ability to control their emotions, make decisions, and interact with people. Ironically, the lonelier people were, the less well they functioned with others.

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Experts estimate that one-fifth of Americans define themselves as lonely (a number that increases to about 35 percent, according to AARP, for people over age 45). About 20 percent of adults say they have only one friend to talk to about important, intimate things, and another 25 percent say they have no one. Even though loneliness is so common, though, people often find it embarrassing to admit. The lonely, despite all their company, feel stigmatized as unlovable, awkward, and socially isolated.

But many of the stereotypes we hold about loneliness turn out to be wrong.

For one thing, lonely people are no lower-status than anyone else. Research conducted in 2000 found that among more than 2,500 undergraduates at Ohio State University, those who called themselves lonely had just as much "social capital" — defined by physical attractiveness, height, weight, socioeconomic status, and academic achievement — as their non-lonely peers.

For another thing, lonely people are not necessarily more isolated. The students at Ohio State who were lonely belonged to as many clubs and had as many roommates as those who were "socially embedded." And while some studies indicate that living alone puts people at greater risk for loneliness, living with a spouse is not necessarily any protection. In fact, researchers at the University of California, San Francisco, reported in 2012 that among nearly 700 Americans over age 60 who described themselves as lonely, 62.5 percent were married. (As a comparison, 72 percent of men over 65 were married in 2011, but just 42 percent of older women.)

What is different about lonely people is how they interpret their interactions with friends and acquaintances. In the Ohio State study, lonely people tended to feel put upon and misunderstood. They were, the researchers wrote, "more likely to attribute problems in social relationships to others," and to see themselves "as victims who are already giving as much as they can to their relationships."

In other words, people grow lonely because of the gloomy stories they tell themselves. And, in a cruel twist, the loneliness itself can further distort their thinking, making them misread other people's good intentions, which in turn causes them to withdraw to protect themselves from further rejection — and causes other people to keep them at arm's length.

According to Guy Winch, a New York psychologist and author of Emotional First Aid, lonely people can become "overly defensive and come across to others as detached, aloof, or even hostile — which only pushes them further away." Loneliness can create its own self-defeating behavior.