In 2001, neuroscientist Robert Provine and his team observed over 1,200 people spontaneously laughing in different locations and noted several interesting things:
Laughter is found in humans of all cultures.
People can't laugh on command without it being fake.
People laugh 30 times more often when with others
Laughter is contagious.
Babies show laughter in as early as 4 months
Only 20% of laughter is from jokes! Most actually follows normal conversations and actually bonds people socially.
People tend to laugh at the end of sentence rather than in the middle.
Women laugh more than twice as much as men.
The higher up in social status you are the less you will laugh.
Slide 3
It's not just humans!
Chimps have been found to tickle each other and laugh, even if its just pretend.
Neuroscientist Jaak Panksepp have found that rats can laugh on a sub sonic level when tickled. Check out a video clip of it here.
You don't need jokes to get people to laugh.
If you want people to laugh, laugh yourself.
When using or designing for online communication keep in mind that synchronous communications (i.e. instant messaging, video conferencing, webcams, etc.) that allow for laughter are more likely to lead to social bonding rather than the more prominent asynchronous communications used.