Robert Povine is one of the few neuroscientists that studies laughter. He's spent many hours observing when and why people laugh. Him and his team observed 1,200 people laughing in different locations. This is what they found:

Laughter is universal. All humans in all cultures laugh.
Laughter is unconscious. People can't actually laugh on command - it will be fake laughter if they try.
Laughter is for social communication. People rarely laugh when they're alone. They laugh 30 times more often when they're with others.
Laughter is contagious. People will smile and then start laughing as they hear others laugh.
Laughter appears early in babies at about four months old.
Laughter isn't about humor. Naturally occurring laughter often followed statements such as "Hey John, where ya been?". Laughter after these types of statements bonds people together socially. Only 20% of laughter is from jokes.
People rarely laugh in the middle of a sentence. It's usually at the end.
The person who is speaking laughs twice as much as the person who is listening.
Women laugh more than twice as much as men.
Laughter denotes social status. The higher up on the hierarchy you are in a group, the less you will laugh.
(These statements are taken directly from "100 Things Every Designers Needs to Know About People" by Susan M. Weinschenk, Ph.D. and are not my own words.)