Washington ? So a scientist walks into a shopping mall to watch people laugh.

There’s no punchline. Laughter is a serious scientific subject, one that researchers are still trying to figure out.

Laughing is primal, our first way of communicating. Apes laugh. So do dogs and rats. Babies laugh long before they speak. No one teaches you how to laugh. You just do. And often you laugh involuntarily, in a specific rhythm and in certain spots in conversation.

You may laugh at a prank on April Fools’ Day. But surprisingly, only 10 to 15 percent of laughter is the result of someone making a joke, said Baltimore neuroscientist Robert Provine, who has studied laughter for decades. Laughter is mostly about social responses rather than reaction to a joke.

“Laughter above all else is a social thing,” Provine said. “The requirement for laughter is another person.”

Over the years, Provine, a professor with the University of Maryland Baltimore County, has boiled laughter down to its basics.

“All language groups laugh ‘ha-ha-ha’ basically the same way,” he said. “Whether you speak Mandarin, French or English, everyone will understand laughter. … There’s a pattern generator in our brain that produces this sound.”

Each “ha” is about one-15th of a second, repeated every fifth of a second, he said. Laugh faster or slower than that and it sounds more like panting or something else.

Deaf people laugh without hearing, and people on cell phones laugh without seeing, illustrating that laughter isn’t dependent on a single sense but on social interactions, said Provine, author of the book “Laughter: A Scientific Investigation.”

“It’s joy, it’s positive engagement with life,” said Jaak Panksepp, a Bowling Green University psychology professor. “It’s deeply social.”

And it’s not just a people thing either. Chimps tickle each other and even laugh when another chimp pretends to tickle them.

“That’s my candidate for the most ancient joke,” Provine said. “It’s a feigned tickle. That’s primal humor.”