Here's an experiment: Try to laugh out loud, right now. Did you find it difficult? Laughter is very difficult to fake, so when we do let loose with gaggles of giggles, it's a largely involuntary action. Now think about this: Do you find yourself laughing more at films when you see them in a theater with other people, or when you're watching them at home alone on television? If you're like most people, you'll laugh more at a funny film that you see with an audience. Researcher Robert Provine has found that laughter is 30 times more frequent in groups compared to private settings [source: Provine]. That's not to say we don't find things funny when we're by ourselves, but we're more likely to smile or talk to ourselves than to roll on the floor laughing [source: Provine].
Because laughter is an involuntary action most often occurring in groups, Provine theorized that our brains likely have a laugh detector that triggers some sort of laugh generator. Researchers at University College London may have found that neural mechanism. In a 2006 study, the researchers played subjects a host of sounds while monitoring their brain's responses with a functional magnetic resonance imaging (fMRI) scanner. The subjects were played a mix of positive sounds, such as laughter and a friendly shout, as well as negative sounds, which included retching and screaming.
All of the sounds activated a part of the brain known as the premotor cortical region; this part of the brain readies our facial muscles to react to sounds. In other words, when people heard laughing, they began to smile. But don't worry; people didn't start retching as well -- responses for the negative sounds were lower, indicating that our brain is much more likely to respond to positive sounds than to negative ones [source: Thompson].
If you're worried that your brain is forcing you to laugh at things that aren't funny, consider what a boon this was to our ancestors. If laughter predated speech, as some theorists suggest, then this penchant for positivity was an important way to demonstrate friendliness. Laugher was a way to show that you meant another group no harm, that you wanted to belong. Even now, laughing is an important social tool that builds bonds between people. A laugh is a way to encourage conversation on a first date and a means to bring people together over a shared experience. Thus, it appears that the old expression is true -- laugh, and the whole world laughs with you.