An interesting study on how laughing and pain can be correlated was conducted by Robin Dunbar, professor of evolutionary psychology at Oxford. The study emphasizes how entwined our bodies and emotions can be. Most of us probably think of laughter as a response to something funny - as an emotion. But laughter is fundamentally a physical action. ''Laughter involves the repeated, forceful exhalation of breath from the lungs,'' said Dunbar. ''The muscles of the diaphragm have to work very hard.'' We've all heard the phrase ''laugh until it hurts'', he points out. That pain isn't metaphoric; prolonged laughing can be painful and exhausting. Rather like a difficult workout.

Dunbar and his colleagues had their volunteers watch, both alone and as part of a group, a series of short videos that were either comic or drily factual documentaries. But first the volunteers submitted to a test of their pain threshold, as determined by how long they could tolerate a tightening blood pressure cuff or a frozen cooling sleeve.

The decision to introduce pain into this otherwise fun-loving study stems from one of the better-established effects of strenuous exercise: that it causes the body to release endorphins, or natural opiates. It's difficult to study endorphin production directly, however, since much of the action takes place within the working brain and requires a lumbar puncture to monitor. That is not a procedure volunteers willingly undergo, particularly in a study about laughing. Instead, Dunbar and his colleagues turned to pain thresholds, an indirect but generally accepted marker of endorphin production. If someone's pain threshold rises, he or she is presumed to be awash in the natural analgesics.

And in Dunbar's experiments, pain thresholds did go up after people watched the funny videos, but not after they viewed the documentaries. The only difference between the two experiences was that in one, people laughed, a physical reaction that the scientists quantified with audio monitors. They could hear their volunteers’ belly-laughing. Their abdominal muscles were contracting. Their endorphin levels were increasing in response, and their pain thresholds and general sense of enjoyment were on the rise.

In other words, it was the physical act of laughing, the contracting of muscles and resulting biochemical reactions that prompted at least in part, the pleasure of watching the comedy.

Why the interplay of endorphins and laughing should be of interest to those of us who exercise may not be immediately obvious. But as Dunbar points out, what happens during one type of physical exertion probably happens in others. Laughter is an infectious activity. In this study, people laughed more readily and lustily when they watched the comic videos as a group than when they watched them individually, and their pain thresholds, concomitantly, rose higher after group viewing. With laughter, as with exercise, it seems, there really is no gain without some element of pain.

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