We can tell when sweat comes from someone terrified or happily exercising, detect ovulation in the pitch of a woman’s voice and discern dominance status by glimpsing photos of people for a 20th of a second. A new paper has just revealed another way our brains keep track of the social scene.

The research concerns what the paper calls “neuromechanical oscillations involving rhythmic laryngeal and superlaryngeal activity”—that is, laughter. Of course, laughter can signal many different things, including attempted wooing, warm affiliation, social dominance (a derisive guffaw) and subordination (an obsequious titter).

A particularly interesting type is “co-laughter,” two people laughing together simultaneously. It is more common among women than men, more frequent among friends than strangers. And co-laughter turns out to contain useful social information.

Writing in April in the Proceedings of the National Academy of Sciences, Greg Bryant of the University of California, Los Angeles, and more than 30 co-authors from around the world did a fascinating cross-cultural study. The researchers prompted college volunteers with topics likely to produce laughter (such as bad roommate experiences) and then had pairs of the students chat. The researchers recorded the conversations and identified instances of co-laughter. Crucially, half the pairs were friends and half were strangers.

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