This video explores the science behind laughter, examining its physical effects on the body and its evolutionary origins. It discusses how laughter may have developed as a social signal among humans and other animals, and why it is often contagious.

Isn't it odd that when something's funny, you might show your teeth, change your breathing, become weak and achy in some places, and maybe even cry? In other words, why do we do this bizarre thing that is laughter? When you laugh, your abdominal muscles contract rapidly. This alters your breathing patterns, increasing the pressure in your chest cavity and pushing air out, which might audibly emerge as a snort, wheeze, or vocalization. Because you're exerting your abdominal muscles much more than you usually would while talking, they may start to hurt. Laughter also inhibits your reflexes and muscle control, causing sensations like leg weakness. So where does this funny phenomenon come from? Because there's no archaeological record of laughter, it's impossible to say exactly how and why it evolved. But scientists have some theories. Importantly, humans are not the only animals today that do something like laughter. Using ultrasonic recorders, researchers in the late 90s realized that rats were basically giggling while being tickled. Scientists have since compiled evidence of at least 65 species—mostly mammals, but also some birds—that vocalize during social play. Some, unsurprisingly, are our closest relatives. By recording and analyzing the sounds primates make while playing and being tickled, researchers grew more convinced that the ancient ancestor of all great apes did something like laughter. And because other apes make laughter-like sounds during rough-and-tumble play, they think laughter may have originally developed to clearly signal friendly, non-aggressive intent. But of course, humans don't just laugh when we're wrestling, but also when we're amused and even surprised, confused, or nervous. Some scientists think laughter took on expanded functions after humans split from other great apes and developed large social groups and more complex language abilities. They hypothesize that laughter gradually became something we could use not just during play, but within speech to convey subtle meanings and a range of contexts to show our emotions. This is thought to be one of the reasons that laughter is contagious. It's like an invitation to share in someone's emotional state. Just hearing clips of laughter can activate key regions in your brain, triggering you to smile or laugh yourself. And when participants in one study watched a funny video, they laughed significantly longer and more often when another person was present, even though they reported feeling the same level of amusement. Human laughter is also generally louder than the play vocalizations of most animals.