Of all the things about human behavior, there’s perhaps nothing more apparent and universal as laughter — and by extension, smiling. Yet, despite how much we all laugh, we’re still very much in the dark about why we do it.

In a five-part series from The Atlantic, Dr. Robert Provine, a neuroscientist at the University of Maryland, Baltimore County, offers some insights into why we laugh. The professor, who has been studying laughter for the better part of two decades now, says it’s an involuntary behavior that likely evolved as we did. He points out that chimpanzees also laugh, albeit in a different pattern, and that humans’ own laughing patterns are distinct.

The way that “ha-ha-ha” comes out is more unique than many of us realize. “The chimpanzee version [of a laugh] is an in-and-out sound,” Provine says in the video. “In humans, it has been transformed into a parsed exhalation.”

That exhalation is what makes a real laugh. Studies have shown we’re able to discern between fake laughs and real ones because of the way they come out of the mouth. As Provine describes, laughs come out in a pulsitive pattern, characterized by short exhalations that last about a fifteenth of a second and repeat about every fifth of a second. Fake laughs often don’t include these exhalations. “We’re neurologically programmed to laugh in a particular way,” Provine said. Any other way, and “it doesn’t sound like laughter anymore.”

While laughter has many positive effects, from staving off memory loss to improving wellbeing, not all laughter is positive. To learn more about what Provine has discovered about laughter, check out The Atlantic’s video below.