Monkey See, Monkey Do, Monkey Connect (Science Writing) by Frans de Waal

What intrigues me most about laughter is how it spreads. It’s
almost impossible not to laugh when everybody else is.
There have been laughing epidemics, in which no one could stop
and some even died in a prolonged fit. There are laughing churches
and laugh therapies based on the healing power of laughter. The
must-have toy of 1996—Tickle Me Elmo—laughed hysterically
after being squeezed three times in a row. All of this because we
love to laugh and can’t resist joining laughing around us. This is
why comedy shows on television have laugh tracks and why theater
audiences are sometimes sprinkled with “laugh plants”: people paid
to produce raucous laughing at any joke that comes along.
The infectiousness of laughter even works across species.
Below my office window at the Yerkes Primate Center, I often hear
my chimps laugh during rough-and-tumble games, and I cannot
suppress a chuckle myself. It’s such a happy sound. Tickling and
wrestling are the typical laugh triggers for apes, and probably
the original ones for humans. The fact that tickling oneself is
notoriously ineffective attests to its social significance. An when
young apes put on their play face, their friends join in with the
same expression as rapidly and easily as humans do with laughter.