system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does laughter "infect" other people?
gpt
So imagine you're an ape (not that hard to imagine, considering...you **are** one, but...go back a bit. Go back half a million years). Your friends and family are sitting around in the branches of a tree. And you're sitting there munching on some weird piece of fruit-thing. It tastes like ass...but you've not invented language yet, so you can't complain.

When suddenly you hear a noise in the bush below.

You stop munching and watch. What's in the bush? A predator? A prehistoric big-cat of some kind? You let out an alert howl. And all your fellow apes stop eating and repeat the howl. Very quickly the whole troop is alerted to danger, and dozens of eyes start scanning the area with their front facing, colour-vision eyes.

Then from the bush slinks a harmless skunk, going about it's business. You let out a guttural vocalization. A whoop of relief. It's quickly picked up by the rest of the troop and repeated. It's not a laugh as we would understand it, but it functions the same way. It signals...relief, safety, humor. The absence of danger.

Basically, laughter is one of those primitive things that exists in our brain. Evolution has taught us the value of group vocalizations to signal distress, pain, sorrow, happiness. As social queues to help us understand our situation and the status of our family members.

And while humour is an abstract concept, laughter is pretty universal among primates. **It's a signal to our friends that things are okay.** And our ape brains encourage us to pass on the message.

This is why you feel the urge to laugh when someone else is laughing. This is why you spin round in alarm when someone screams. We are social animals, being social.