Laughter is contagious

A neuroscientist confirms it, in an interview with Andrew Anthony in the British newspaper The Guardian, December 24, 2017 (bold, links in original):

Sophie Scott is a senior fellow at University College London. She is an expert in cognitive neuroscience, particularly in relation to communications. This year, she is giving the Royal Institution Christmas lectures looking at how evolution has shaped our bodies to communicate with each other. She also does standup comedy...

...Many animals laugh, including, apparently, rats. What have rats got to laugh about?
They laugh for the same reasons as other animals: it’s a social behaviour. We tend to associate laughing with jokes and humour in adult humans, but actually laughter is always something that happens primarily around members of your own species. Particularly the ones you know, particularly the ones you like. Rats laugh when they’re tickled and when they’re playing. That’s true of apes and humans too.

What are the advantages of laughter over a wry smile?
They are twofold. Laughter is a very strong cue to others to join in. So it works as a behaviourally contagious phenomenon. It also feels good to laugh. You get a kick from laughter. It’s having an effect at multiple levels. There’s some very good research from Robert Levenson on positive affect. If both members of a couple laugh or smile, they can deal with stressful situations. But only if they both do it.

We’re 30 times more likely to laugh with someone than on our own. Is laughing alone an activity that should be of concern?
All laughter’s good. Don’t worry about laughing on your own. It’s a statistical phenomenon – you’re much more likely to do it around other people. I realised the other day the same is true of speaking. You’re much more likely to talk around other people.