Lonely people have less activity in a part of the brain that normally lights up in association with reward, scientists have found. It's not clear if social isolation diminishes the brain-reward response, however, or if people with less activity in that part of the brain tend toward loneliness.

More research will be need to sort out the findings, which come from a study of just 23 female college students. But the finding offers hope that scientists may improve their understanding of loneliness, a growing emotional problem in an increasingly scattered society and one known to raise the risks of several health problems.

The subjects were surveyed with standard questions to determine who felt socially isolated, or lonely, vs. those who did not. They then underwent fMRI brain scans while looking at photos of people enjoying themselves.

The ventral striatum — a region of the brain known from other research to light up in association with rewards such as food and money — was much less activated in the lonely group.

"Given their feelings of social isolation, lonely individuals may be left to find relative comfort in nonsocial rewards," said John Cacioppo, a professor of psychology at the University of Chicago.

Growing problem

About one if five Americans experience loneliness, Cacioppo said. And it is a growing problem in modern society in part because the average household size is decreasing. By 2010, 31 million Americans — roughly 10 percent of the population — will live alone, Cacioppo and his colleagues say.

Previous work has suggested it can be as detrimental to health as smoking, Cacioppo said. In his book, "Loneliness: Human Nature and the Need for Social Connection" (W.W. Norton, 2008), he presented evidence that loneliness is related to less blood flow through the body, poorer immune systems, increased levels of depression and a faster progression of Alzheimer's disease.

A 2006 study by a different research team, of people age 50 to 68, found that those who scored highest on measures of loneliness also had higher blood pressure, a major risk factor for heart disease. The potentially deadly health effect of loneliness accumulates gradually and faster as you get older, that study found.

Although loneliness may be influence brain activity, the research also suggests that activity in the ventral striatum may prompt feelings of loneliness, said Cacioppo's colleague Jean Decety, a professor of psychology and psychiatry at the university. "The study raises the intriguing possibility that loneliness may result from reduced reward-related activity in the ventral striatum in response to social rewards," Decety said.

The results are published in the current issue of the Journal of Cognitive Neuroscience. Cacioppo presented the findings today at the annual meeting of the American Association for the Advancement of Science, in Chicago.

What to do

In his 2008 book, Cacioppo and co-author William Patrick, former science editor at Harvard University Press, argue that loneliness creates a feedback loop that reinforces social anxiety, fear and other negative feelings. Getting out of the loop requires first recognizing it and overcoming the fear related with connecting with others.

"The process begins in rediscovering those positive, physiological sensations that come during the simplest moments of human contact," Patrick said. "But that means overcoming the fear and reaching out."

"Lonely people feel a hunger," Cacioppo added. "The key is to realize that the solution lies not in being fed, but in cooking for and enjoying a meal with others."