1. Contrary to popular belief, the number one catalyst for laughter isn't a joke: It's interacting with another person.
2. That's because the modern-day ha-ha! probably evolved as a form of communication. Our primate ancestors used a similar sound—a sort of pant-pant—to reassure one another that their rough-and-tumble play was all in good fun and not an attack, says Robert R. Provine, a professor of psychology at the University of Maryland, Baltimore County, the author of Curious Behavior, and one of the foremost experts on laughter.
3. One of Provine's earliest experiments proved that just listening to recorded laughing could evoke fits of giggles in subjects (which is why television studios use laugh tracks on sitcoms). In fact, according to his research, you're 30 times more likely to laugh when someone else is around than when you're by yourself.
4. The ideal number of words in a joke? 103.
5. "There is no magic formula or key for what's funny," says Scott Weems, Ph.D., a research scientist at the University of Maryland, College Park, and the author of Ha! The Science of When We Laugh and Why. But, in general, he says, what often makes us laugh is when our brain is expecting one thing and then, in the space of a few words, that expectation is turned on its head. Take the classic Groucho Marx joke: "One morning I shot an elephant in my pajamas. How he got in my pajamas, I don't know."
6. Ten to 15 minutes of daily laughing burns 10 to 40 calories.
7. Our appreciation for the unexpected starts as early as infancy, although on a very basic level. "Parents will notice that they can elicit a giggle from their baby by making a funny face, talking in a funny voice, or playing peekaboo," says Merideth Gattis, Ph.D., a psychologist at Cardiff University, in Wales.
8. British psychologist Richard Wiseman, Ph.D., the author of Quirkology, has revealed clear regional preferences for what we find funny. Americans often like jokes that include a sense of superiority. (Texan: "Where are you from?" Harvard grad: "I come from a place where we do not end our sentences with prepositions." Texan: "OK, where are you from, jackass?") Europeans tend to laugh at jokes that make light of anxiety-provoking topics, like marriage and illness. (A patient says, "Doctor, last night I made a Freudian slip. I was having dinner with my mother-in-law and wanted to say, 'Could you please pass the butter?' But instead I said, 'You silly cow. You have completely ruined my life.'") And Brits? Wiseman finds that they are tickled most by wordplay. (Patient: "Doctor, I've got a strawberry stuck up my bum." Doctor: "I've got some cream for that.")
9. An adult laughs an estimates 15 to 20 times a day.
10. "The same pleasure sensors in the brain that are activated when we eat chocolate become active when we find something funny," says Weems. "It's a natural high." In fact, a 2003 brain-scan study published in the journal Neuron found that the dopamine reward centers and pathways in the brains of subjects lit up when they were treated to a funny cartoon, but not when they were shown an unfunny version.
11. Research has linked laughter with boosts in immune function, pain tolerance, cardiovascular health and maybe even memory retention.
12. A typical 10-minute conversation has an average of 5.8 bouts of laughter.
13. Even those with zero sense of humor can reap the benefits of laughter. How? Fake it. A 2002 study in Psychological Reports reveals that forcing yourself to laugh (or even just to smile) can improve your mood. The human brain is not able to distinguish spontaneous laughter from self-induced; therefore the corresponding health-related benefits are alleged to be alike, according to a 2010 report in Alternative Therapies in Health and Medicine by Ramon Mora-Ripoll, M.D., Ph.D., an advisory board member of the Laughter Online University, a supplier of online laughter education.