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

This joke is greeted with groans that resonate through a warehouse in London.

We're at a joke-testing session with a firm that produces supplies for gatherings. Its catalogue features Christmas crackers.

The company's founder grins, almost apologetically at the joke. But the pun has been selected and will feature in future crackers.

"The success is gauged by the joke by the volume of groans and the intensity of the groans around the table," the founder says.

The key to a great Christmas cracker pun is not the identical as a stand-up gag per se. It is all about the context - in this case, the communal amusement of the Christmas meal with elders, children and possibly friends.

"You want the joke to be something that unites the child together with the grandparent," she adds.

The Neuroscience Behind Shared Laughter

Gathering to experience shared amusement is not only ancient, experts argue, it is likely to be older than humanity.

"Therefore when you are chuckling with people at the holiday table you are dropping into what's almost certainly a truly ancient mammalian play vocalisation," says a professor.

Shared amusement, she says, aids in forge and strengthen social connections between individuals.

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

"The people you talk to, and laugh with, it leads to enhanced amounts of endorphin uptake," the professor continues.

These natural chemicals are the body's "happy chemicals" and are produced both to reduce tension and discomfort and in reaction to pleasurable experiences, such as chuckling with loved ones over a particularly awful festive cracker gag.

"You're not just chuckling at a silly joke with a holiday cracker," the expert states. "You are actually performing a lot of the really important task of making, maintaining the social bonds you have with the people you care about."

Which Occurs In the Mind?

But what is truly taking place within the brain when we listen to a gag?

An awful lot occurs in response to comedy, it transpires.

Using functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which parts of the brain are more active, scientists have been able to map the regions that get more blood flow.

The research entails scanning the minds of volunteer participants and then exposing them to a collection of humorous words, accompanied by either a non-emotional sound, or pre-recorded laughter.

"During the study we got a really fascinating pattern of neural activity," says the professor.

A gag activates not just the parts of the mind in charge of auditory processing and understanding language, but also brain regions associated with both planning and starting motion and those involved in vision and memory.

Combine all of this as a whole, and people listening to a joke have a sophisticated set of brain reactions that underpin the amusement we hear.

The Infectious Power of Chuckles

Researchers discovered that when a humorous phrase is paired with chuckles there is a greater response in the mind than the same phrase when accompanied by a neutral sound.

"This activation occurred in parts of the mind that you would use to move your face into a smile or a laugh," she says.

It means people are not just responding to humorous jokes, they are reacting to the laughter that accompanies them.

Amusement, says the professor, can be infectious.

So what does this imply for the chuckles found around a holiday gathering?

"You laugh harder when you are familiar with others," she notes, "and laughter increases further when you like them or love them."

When it comes to Christmas cracker jokes, she says, the feel-good factor is more likely to be triggered not by the joke in itself, but from the reaction to it.

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

The Quest for the Perfect Cracker Joke

Will we ever discover the ultimate gag?

Likely not, but that has not prevented experts from attempting to.

Years ago, a psychologist set up a scientific search for the planet's most humorous gag.

More than tens of thousands of gags later, with scores provided by hundreds of thousands of participants around the world, he has a better idea than many as to what works and what does not.

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

"They must also need to be bad jokes, puns that make us moan," he adds.

The more "awful" the joke, he says the better.

"This is because if nobody finds it funny – it's the joke's shortcoming, not your own.

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

"It creates a shared experience at the gathering and I think it's wonderful."