Although a student may have 400 friends on Facebook, recent studies are showing close friendships among Americans are declining, especially among men.

This social isolation can strongly affect an individual's mental and physical health, researchers say.

"Being lonely could lead to a compromised immune system via depression," said Peter Arnett, associate professor of clinical psychology at Penn State. He said some people don't mind spending time on their own and don't need as much contact, but for people who do need it, living with loneliness can be stressful and lead to long-term health problems.

In a survey done in 1985, on average, Americans said they had three close friends that they could confide in, but in the same survey done in the summer, Americans said they had only two. The number of people who said they have no one to discuss important matters with has doubled to one in four, according to the June issue of the American Sociological Review.

The study found that two groups of men -- high-school educated whites in their 20s and 30s and blacks aged 60 and older -- experienced more significant drops in the size of their main group of friends.

"We've argued women are more sensitive and more likely to pay attention to social networks, but men may not. Men are fairly oblivious to the social networks," said psychologist Sheldon Cohen of Carnegie Mellon University and author of several immunity studies.

In his immunity study, Cohen said he evaluated first-year college students who were lonely or had a small circle of friends and found they had a weaker immune response to a normal flu vaccine. He also found that young and middle-aged adults who had many different types of strong social connections were less susceptible to cold infections.

"People live longer who are more socially integrated and are less likely to get colds and diseases," said Cohen.

Although having many friendships alone isn't enough to protect people, Cohen said having one close source of social support, such as a spouse, is enough.

"For older men, they come to rely on important women in their lives, like wives, in encouragement to see a physician and eating healthier," said David Almeida, associate professor of human development at Penn State. "If men don't have this, their health declines."

He also said he thinks much of the reason people tend to have fewer confidantes may be because of all the technology we have now.

"Technology may hurt in some ways; people are less likely to talk to friends face-to-face, but helpful in that it does allow you to stay connected with people far away," he said.

The types of social interactions change as people grow older and tend to focus on more emotional attachments with closer friends, he said.

"When you're younger, you want lots of friends, but as you get older you get more choosey," Almeida said.