Laughter, moreover, is not limited to humans. Anyone who has spent time watching chimpanzees or bonobos will recognize that evolution has provided the same capacity for our two closest relatives, although chimp and bonobo laughter is more breathy, less vocally overt than our own. It sounds rather like someone frantically sawing wood. People who study gorillas and orangutans inform me that these two ape species laugh as well. So we might conclude that laughter evolved in a common ancestor of the four great apes . . . but in fact play accompanied by a smilelike facial expression ("playface") is widely distributed among mammals, while, according to neuroscientists Jaak Panksepp and Jeff Burgdorf, even rats may be capable of laughter.

Panksepp and Burgdorf had spent some time watching their laboratory rats play before asking themselves whether there was an auditory element to the rats' play that experimenters couldn't hear. They asked the question experimentally by deafening some rats and then observing their play, discovering in that fashion that being deaf moderately reduced the intensity of play. This implied not only that there was an auditory component to the play, but that such a component encouraged playfulness.

Since the auditory event was clearly happening outside the range of human hearing, the researchers brought in special equipment to register ultrasound. Eavesdropping thus, they discovered that the rats' play was accompanied by chirping done in the range of 50 kilohertz. These chirpy vocalizations, it turned out, were particularly abundant just before play began, as if they were associated with anticipation or encouragement.

Panskepp and Burgdorf decided that the high-pitched chirping might best be described as a "laughter-type response," and they began to pursue that concept further. In the spring of 1997, Panksepp arrived at the laboratory one day and said to his colleague: "Let's go tickle some rats." They tickled rats and found that the 50 kilohertz chirps were being emitted at more than double the rate ordinarily recorded during play. They also discovered that the animals sought out tickling, and that tickling stimulated them in their own play. The scientists decided, as they would later phrase it, "to remain open to the possibility that there was some type of ancestral relationship between this response, and the primitive laughter that most members of the human species exhibit in rudimentary form by the time they are three months old."

It was a fascinating observation, but a number of Panskepp and Bergdorf's scientific peers didn't think so. As a result, the observations on rat laughter were first reported in the popular media, featured in a BBC program and another on the Discovery channel. Following that popular exposure, Panskepp received many letters from rat-enthusiasts, including one young woman from California who, after seeing the Discovery show, tickled her young son's pet rat, Pinky. It took only a week before Pinky was completely conditioned to expect tickling, and occasionally the woman would be able to hear the high-pitched chirping. "It's been about 4 weeks," she wrote, "that I have been tickling him every day and now, the second I walk into the room, he starts gnawing on the bars of his cage and bouncing around like a kangaroo until I tickle him." Her rat preferred being tickled to eating, she went on, and she noted that he would turn over and expose an especially ticklish spot on his stomach. "It's the funniest thing I've ever seen, even though my family thought I had lost my mind until I showed them."

The researchers went on to identify a number of ways in which 50-kilohertz chirping was comparable to social laughter in young children. It was provoked by play and tickling. There were tickle-sensitive parts of the body. As the rats grew older, they were less susceptible to being tickled. There was a negative relationship between unpleasant experiences and the capacity to chirp, and a positive one between chirping and play. Tickling was more attractive to rats with higher levels of 50-kilohertz chirping, while rats who exhibited more abundant chirping and a greater response to tickling were more popular--that is, more likely to be chosen as social partners--than other rats.

But was it really laughter? Panksepp and Burgdorf's own response to some of their colleagues' reflexive fear of anthropomorphism concerning the observation of laughing rats might be summarized like this. First, no one so far had come up with a better assessment of the phenomena they were describing. Second, it seemed to make perfect evolutionary sense to believe that "affective processes" could be found in the brains of mammals other than humans. And third, if indeed they were witnessing an animal homolog to human laughter--in other words, that these 50-kilohertz chirps were "the sounds of social joy"--then they had found in their laboratory rats an "excellent animal model to help decipher scientifically one of the great mysteries of human emotions--the primal joyful nature of laughter and positive social interchange."

So even though laughter may not cure your physical ills in the way that Norman Cousins imagined, it might provide a fair treatment for your social and psychological ills. Laugh, the philosopher says, and the world will laugh with you. Laugh, the psychologist says, because laughter could be one of nature's primary social lubricants.