Most of us want in on laughter, wherever we may encounter it in daily life. If you've ever walked into a room full of laughing people, your first instinct was probably to search for a visible source of hilarity. You may have asked someone to explain the joke. Of course, people were probably too busy laughing to explain it.

More often than not, there is no joke it all, and everyday laughter is rarely about comedy. In the words of psychiatrist Robert Provine, laughter is a "social vocalization that binds people together." It does this in a variety of ways. Laughter can serve as a simple expression of recognition -- for example, the "oh, I know what you mean" laugh that we often share to create a moment of mutual understanding. We laugh courteously, apologetically, self-consciously, and often for completely mysterious reasons. But we rarely laugh when we are by ourselves.

Laughter can come into existence with only the smallest pretext of shared experience between two people. There is nothing particularly funny about the time you unexpectedly bumped into your best friend on the sidewalk, and yet laughter seemed to be the only option at the time. Would you have laughed as hard, or at all, if the person you bumped into was a stranger?

Robert Provine has analyzed this "togetherness" trigger that qualifies most laughter. Provine and his students at the University of Maryland took notes about the everyday laughter they encountered around campus. They recorded information such as the gender of the person laughing, the gender of the person who caused the laughter, and the comments made just before the laughter started.

More often than not, these comments were just unfunny statements on the surface. "It was nice to meet you, too!" and "Have a nice day!" for example, triggered laughter. Jokes only occurred rarely. Often it was the speakers who laughed after their own comments to others. As you might guess, the students did not find very many solitary people laughing to themselves. Provine concluded that "the critical stimulus for a laugh is another person, not a joke."

The Physical Effects of Laughter

Strong social connections have been shown to safeguard physical health, especially as we age. The benefits we derive from laughter are similar to the benefits we get from social interaction.

Here are a few examples of the physical effects of laughter:

-- When we laugh, we release feel-good endorphins that have been found to reduce physical pain. Journalist Norman Cousins, after being hospitalized for a spinal condition, incorporated laughter into his personal recovery program. He found that watching the Marx brothers every night made him laugh so much that afterward he was able to sleep for at least two hours without feeling any pain.

-- Laughter inhibits the stress hormone, cortisol, which has adverse effects on immune functioning. Laughter causes the immune system to produce more T cells, immune proteins and antibodies.

-- By increasing heart rate, laughter benefits our cardiovascular system in a way similar to exercise. In one study, 300 participants split into two groups that watched two different movies. Half of the group watched a comedy ("There's Something About Mary") while the other half watched a drama ("Saving Private Ryan"). The group that watched the comedy had a 30-40 percent increased dilation in their blood vessels compared with the group who watched the drama.

Reduced Risk of Heart Disease

Cardiologists at the University of Maryland Medical Center in Baltimore have found that people with heart disease were less likely to have a sense of humor than their heart-healthy contemporaries. Of the 300 participants surveyed, half had healthy hearts while the other half had a history of heart disease. The participants answered questions examining the extent to which humor played a role in their everyday lives. In particular, those who laughed or used humor to cope with stressful situations were less likely to have heart disease.

Dr. Michael Miller, one of the cardiologists involved in the study, believes that incorporating laughter into a daily routine might be as important for heart health as diet and exercise. There are numerous "laughing groups" operating in many U.S. cities with the aim of spreading the benefits of laughter to their practitioners. There is even an entire branch of Yoga devoted to communal laughing. Led by a certified instructor, participants of Laughter Yoga get together and, well, laugh. The atmosphere builds on the infectiousness of laughter and playful behavior. Even if you went into the session in a less than elated mood, chances are by the end, your laughter will have evolved into something genuine.

"Laughter is the shortest distance between two people."

Adam Johnson writes for Youville Assisted Living Residences, member of Covenant Health Systems, a Catholic, multi-institutional health and elder care organization serving New England.