"You got to have friends to make that day last long," sings Bette Midler.

Good friends may help your life last longer, too.

A recent study followed nearly 1,500 older people for 10 years. It found that those who had a large network of friends outlived those with fewer friends by more than 20%.

Why? Some think good friends keep you from doing things that are bad for you, like smoking and heavy drinking. Friends may also ward off depression, boost your self-esteem, and provide support.

As people age, they tend to be more selective in their choice of friends, so they spend more time with people they like.

Close relationships with children and relatives, in contrast, had almost no effect on longevity. Lynne C. Giles, one of the researchers who conducted the study, emphasized that family ties are important, they just seem to have little effect on survival.

The Health Benefits of Good Friends

Lots of research has shown social support and good health are connected.

One recent study of people with ovarian cancer says those with lots of social support had much lower levels of a protein linked to more aggressive cancers. This made their chemotherapy treatments more effective.

In another study, women with breast cancer in a support group lived twice as long as those not in a group. They also had much less pain.

Sheldon Cohen, PhD, a psychology professor at Carnegie Mellon University in Pittsburgh, says strong social support helps people cope with stress.

"There may be broader effects as well,” Cohen says. “Friends encourage you to take better care of yourself. And people with wider social networks are higher in self-esteem, and they feel they have more control over their lives."

Other studies show people with fewer friends tend to die sooner after having a heart attack than people with a strong social network. Having lots of friends may even reduce your chance of catching a cold.

"People with social support have fewer cardiovascular problems and immune problems, and lower levels of cortisol -- a stress hormone," says Tasha R. Howe, PhD, associate professor of psychology at Humboldt State University.

"[We] are social animals, and we have evolved to be in groups,” Howe says. “We have always needed others for our survival. It's in our genes.”

People with a big social group tend to be more at peace, which leads to better health, Howe says.

Friends Can Be Stressful

Your buddies can be a source of stress, though. Friends can cause more stress than others because we care so much about them.

Julianne Holt-Lunstad, PhD, associate professor of psychology at Brigham Young University, says  dealing with people who cause conflicted feelings in us can raise blood pressure more than dealing with people we don't like.

"My colleagues and I were interested in relationships that contain a mix of positivity and negativity," she says. "For example, you might love your mother very much, but still find her overbearing or critical at times."

Holt-Lunstad and her colleagues found that blood pressure was highest when people were interacting with someone they had mixed feelings about.

"We suspect that people we feel positive toward can hurt us that much more when they make a snide comment or don't come through for us because they are important to us,” she says. “Friends may help us cope with stress, but they also may create stress."

So would we be better off having no friends at all? Hardly.

"One thing research shows is that as one's social network gets smaller, one's risk for mortality increases," Holt-Lunstad says.

How much? She says it’s almost as much as if you smoke.

The Impact of Loneliness

What about loners? Are they at greater risk of dying because they like to be alone?

Only if they feel lonely.

Drug use among young people is higher among those who say they’re lonely. Older lonely people tend to have higher blood pressure and poorer sleep quality. They also were more tense and anxious.

In one study, college freshmen who had small social networks and claimed to be lonely had weaker immune responses to flu vaccinations. They also had higher levels of stress hormones in their blood.

How Women's Friendships Are Different

In general, women are better at keeping friends than men. Women "tend and befriend," says Shelley E. Taylor, PhD, a psychology professor at UCLA. They respond to stress by protecting, nurturing, and seeking support from others. This pattern regulates the seeking, giving, and receipt of social support, Taylor says. It reduces psychological and biological stress.

Margaret Gibbs, PhD, professor of psychology at Fairleigh Dickinson University, says men and women relate to others differently throughout life.

"Male friendships are more about helping each other -- mending the lawn mower, that sort of thing,”  Gibbs says. “Women's friendships tend to have a more emotional content -- listening to friends' stories and coming up with helpful solutions."

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