Laughter is one of the least-studied human emotional states but one of the most important, according to University College London cognitive neuroscientist Sophie Scott.
While laughing is linked to comedy and humour, it is primarily a social behaviour and a form of communicating, she says.
“Some of the laughter that happens in conversations is happening just for social reasons like people are simply laughing because they're showing that they're affiliated.
“We also laugh contagiously so you just catch a laugh from someone else, that happens a lot.
“Robert Provine found at any one point in time in a conversation the person who laughs the most is the person who's talking, so they are actually kind of forcing the person they're talking to to show that they agree, they understand, they recognise.”
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It also makes you vulnerable because it loosens your reflexes which might be why we’re more likely to laugh when we’re with somebody we trust, she says.
“We don't just laugh with anybody, anywhere. We're quite picky about the people that we like to laugh a lot with, you know, real, genuine, helpless laughter.
“If I think about the times when I've laughed and I've really laughed and I couldn't stop, that doesn't happen randomly. That pretty much always happens when I'm with a really close friend or more likely when I'm with my partner and our son.
“So maybe you have to feel a bit safe to let yourself get into that really, really vulnerable state.”
Professor Scott sees laughing as a peculiar vocalisation that resembles an “animal call”.
“It's actually a very primitive, very simple way of making a sound.
“All that's happening when you laugh is that your rib cage starts to move in and out really quickly and with very big deflections, and that's giving you those big sort of ha ha ha sounds. Each one of those is just squeezing air out of you.
“It's also like an animal call in that most animal calls are very reactive and emotional, and it is an emotional vocalisation.”
Other mammals tend to have a similar vocalisation which they make when playing to show they’re not being threatening or inappropriate, she says.
“Some guys in the US did an experiment where they devocalised rats so they couldn't make any sound.
“And they still played with other rats … however, when they're playing, the devocalised rats are more likely to get bitten, because they can't make the sound that shows they're still being playful, so their behavior is more open to misinterpretation.”
Thinking of the complexity of that interaction in rats raises questions about how complex it is for humans, she says.
“It's a very, very complex behaviour actually and you see that reflected in the brain responses and people are always trying to work out what laughter means, even if they're just listening to it in a scanner and no one’s told them to do anything with the laughter, you can't ignore it, you want to know what's going on.”
It’s also perhaps more important in our relationships than we might know, she says.
A longitudinal study in the US measured physiological responses of married couples as they talked about a problem in their life and found they responded with things like smiling and laughing to cope.
“You see them get more relaxed, the stress goes away, but they're also, over time, the couples who stay together for longer and are happier in their relationship and very importantly, this only works if both members of the couple laugh.
“It's the sharing of laughter that seems to be really important in those situations, and I think that's very unlikely to be limited to romantic relationships.”
So we should value laughter and maximise our opportunities to laugh with people we care about, she says.
“In the UK, when we went into the first lockdown, it was really scary … it was very, very stressful.
“I said to my partner and our son … right, every day we put away the computers at 5:30 and we sit down and we watch something on television that will make us laugh and that was solely really explicitly so we had a reason to stop working and laugh together and feel better together.
“And at some point, my son will leave home and move away but until that day happens, actually we still do this at the end of every day we sit down and watch something together.”
