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"How much did Father Christmas's sleigh cost? Zero, it was on the house."

This quip is met by moans that resonate through a warehouse in the capital.

This describes a joke-testing meeting with a company that produces products for social events. Its catalogue features Christmas crackers.

The company's owner grins, nearly sheepishly at the joke. But the joke has made the cut and will appear in future crackers.

"You measure the joke by the volume of groans and the loudness of the groans around the table," she says.

The key to a good holiday cracker joke is not the identical as a good joke per se. It is entirely about the context - in this case, the shared laughter of the Christmas dinner table with elders, kids and potentially neighbours.

"The goal is for the gag to be a thing that unites the eight-year-old together with the 80-year-old," she adds.

The Neuroscience Of Shared Laughter

Coming together to experience communal amusement is not only nothing new, scientists say, it is likely to be older than humanity.

"Therefore when you are chuckling with people at the holiday dinner you are dropping into what's very likely a really ancient mammal play vocalisation," explains a professor.

Shared laughter, she says, aids in make and maintain social bonds between individuals.

Researchers have found that a absence of such social exchanges can seriously harm both psychological and bodily well-being.

"The people you converse with, and laugh with, it results in increased levels of 'happy chemical' release," the professor adds.

Endorphins are the brain's "feel-good compounds" and are released both to alleviate tension and discomfort and in reaction to pleasurable experiences, such as chuckling with loved ones over a truly awful Christmas cracker joke.

"It's not simply laughing at a silly joke with a holiday cracker," the expert states. "You are in fact performing a lot of the really important task of building, preserving the social bonds you have with the people you care about."

What Occurs In the Mind?

But what is actually taking place within the brain when we hear a joke?

A tremendous amount happens in response to humour, it transpires.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which parts of the mind are more active, researchers have been able to chart the regions that receive more blood.

Testing entails scanning the minds of healthy participants and then subjecting them to a database of humorous words, accompanied by either a non-emotional sound, or recorded laughter.

"During the study we got a very interesting pattern of activation," says the professor.

A gag activates not just the parts of the mind in charge of auditory processing and interpreting language, but also neural regions associated with both planning and starting movement and those linked to sight and recall.

Combine these elements together, and people hearing a pun have a complex set of brain reactions that underpin the laughter we experience.

The Contagious Nature of Laughter

Researchers found that when a humorous word is paired with laughter there is a stronger reaction in the brain than the same phrase when accompanied by a non-emotional sound.

"This activation occurred in parts of the brain that you would employ to move your expression into a grin or a laugh," the professor explains.

It indicates people are not just responding to humorous words, they are responding to the laughter that accompanies them.

Laughter, says the professor, can be contagious.

So what does this imply for the chuckles found at a Christmas gathering?

"People laugh more when you are familiar with people," she says, "and laughter increases further when you like them or care for them."

When it comes to festive cracker jokes, she explains, the feel-good effect is more likely to be triggered not by the joke in itself, but from the response to it.

"The laughter is key. The gag is the dreadful holiday cracker joke, and it's just a reason to laugh together."

The Quest for the Perfect Cracker Joke

Is it possible to find the perfect gag?

Likely not, but that has not prevented researchers from trying to.

Years ago, a psychologist set up a scientific project for the planet's funniest joke.

Over 40,000 jokes later, with ratings provided by 350,000 participants around the world, he has a better understanding than many as to what succeeds and what does not.

The ideal festive cracker joke must be short, he explains.

"But they also need to be poor gags, puns that cause us to moan," he adds.

The more "terrible" the gag, he states the more effective.

"The reason is that if no-one finds it funny – it's the joke's shortcoming, not your own.

"The fascinating part about the Christmas cracker jokes is that not one person considers them funny.

"It creates a common moment around the table and I believe it's wonderful."