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"How much did Father Christmas's sled cost? Nothing, it was on the house."

This one-liner is greeted with groans that resonate through a storage facility in London.

We're at a joke-testing meeting with a company that produces products for gatherings. Its repertoire features Christmas crackers.

The firm's owner grins, nearly apologetically at the gag. But the joke has been selected and will appear in future crackers.

"The success is gauged by the gag by the number of groans and the loudness of the groans around the table," she says.

The key to a great holiday cracker pun is not the same as a stand-up gag in itself. It is all about the context - in this instance, the shared laughter of the Christmas meal with grandparents, children and potentially neighbours.

"The goal is for the joke to be a thing that brings the eight-year-old in harmony with the 80-year-old," she states.

The Science Behind Communal Laughter

Gathering to enjoy communal laughter is not only ancient, scientists argue, it is probably to be older than humanity.

"Therefore when you are chuckling with others at the holiday table you are engaging in what's very likely a really primordial mammal social sound," explains a neuroscience expert.

Communal amusement, she explains, aids in make and maintain social connections between individuals.

Researchers have discovered that a lack of such social exchanges can significantly harm mental and physical well-being.

"The people you talk to, and share laughter with, it results in enhanced levels of 'happy chemical' uptake," she adds.

Endorphins are the brain's "feel-good compounds" and are released both to reduce stress and pain and in reaction to enjoyable experiences, such as laughing with loved ones over a truly terrible Christmas cracker gag.

"You're not just chuckling at a silly joke with a holiday cracker," the expert states. "You are actually doing a lot of the really vital task of building, preserving the social bonds you have with those you love."

What Happens In the Brain?

But what is truly happening within the mind when we hear a joke?

A tremendous amount occurs in response to comedy, it transpires.

Employing functional magnetic resonance imaging (fMRI), a type of neural imager which shows which areas of the brain are working harder, scientists have been able to chart the regions that receive more blood.

The research involves scanning the brains of healthy participants and then exposing them to a collection of humorous words, accompanied by either a non-emotional sound, or pre-recorded chuckles.

"During the study we got a really interesting activation pattern of neural activity," says the neuroscientist.

A gag activates not just the parts of the brain in charge of auditory processing and understanding speech, but also brain regions involved in both preparation and initiating movement and those involved in vision and recall.

Put all of this as a whole, and people hearing a pun have a complex series of neural responses that underpin the laughter we hear.

The Infectious Power of Chuckles

Researchers discovered that when a funny word is combined with chuckles there is a greater reaction in the brain than the same phrase when accompanied by a non-emotional sound.

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

It means people are not just reacting to funny jokes, they are reacting to the amusement that accompanies them.

Amusement, says the professor, can be contagious.

So what does this imply for the laughter found at a holiday table?

"People laugh harder when you know people," she says, "and you laugh more when you like them or love them."

When it comes to Christmas cracker puns, she says, the positive factor is more probable to be caused not by the gag itself, but from the reaction to it.

"It's the laughter. The gag is the dreadful Christmas cracker pun, and it's just a reason to laugh together."

The Quest for the Perfect Cracker Joke

Is it possible to find the ultimate gag?

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

In 2001, a psychologist set up a research search for the world's most humorous gag.

More than 40,000 gags later, with ratings lodged by 350,000 people globally, he has a clearer idea than most as to what succeeds and what does not.

The perfect Christmas cracker joke needs to be brief, he explains.

"But they also be bad gags, puns that cause us to moan," he continues.

The more "terrible" the gag, he says the better.

"This is because if no-one laughs – it's the joke's shortcoming, not yours.

"What's interesting about the holiday cracker jokes is that not one person find them funny.

"It creates a shared experience around the table and I think it's lovely."