Laughter is regularly promoted as a source of health and well being, but it has been hard to pin down exactly why laughing until it hurts feels so good.

The answer, reports Robin Dunbar, an evolutionary psychologist at Oxford, is not the intellectual pleasure of cerebral humor, but the physical act of laughing. The simple muscular exertions involved in producing the familiar ha, ha, ha, he said, trigger an increase in endorphins, the brain chemicals known for their feel-good effect.

His results build on a long history of scientific attempts to understand a deceptively simple and universal behavior. “Laughter is very weird stuff, actually. That’s why we got interested in it,” Dunbar said. And the findings fit well with a growing sense that laughter contributes to group bonding and may have been important in the evolution of highly social humans.

Social laughter, he suggests, relaxed and contagious, is “grooming at a distance,” an activity that fosters closeness in a group the way one-on-one grooming, patting and delousing promote and maintain bonds between individual primates of all sorts.

The findings, published in the Proceedings of the Royal Society B: Biological Sciences, eliminated the possibility that the pain resistance measured was the result of a general sense of well being rather than actual laughter. And, he said, they also provided a partial answer to the ageless conundrum of whether we laugh because we feel giddy, or feel giddy because we laugh.