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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 storage facility in London.

We're at a humor-evaluation meeting with a firm that makes supplies for social events. Its repertoire includes Christmas crackers.

The company's founder grins, almost 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 moans and the intensity of the groans around the table," she says.

The secret to a good Christmas cracker pun is not the same as a stand-up joke per se. It is entirely about the context - in this case, the shared amusement of the holiday dinner table with elders, kids and potentially friends.

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

The Neuroscience Behind Communal Laughter

Gathering to experience shared amusement is not only ancient, scientists argue, it is probably to be pre-human.

"So when you are laughing with others at the holiday dinner you are dropping into what's almost certainly a truly primordial mammalian social vocalisation," says a neuroscience expert.

Shared laughter, she says, aids in forge and strengthen social connections between people.

Researchers have found that a lack of these social exchanges can seriously damage mental and physical health.

"Those you converse with, and share laughter with, it leads to enhanced amounts of endorphin release," she continues.

Endorphins are the body's "feel-good compounds" and are produced both to reduce tension and discomfort and in response to pleasurable experiences, such as chuckling with loved ones over a particularly awful Christmas cracker gag.

"It's not simply chuckling at a foolish joke with a Christmas cracker," the expert says. "You are in fact performing a lot of the truly vital work of making, maintaining the connections you have with those you care about."

Which Occurs In the Brain?

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

An awful lot happens in response to humour, it transpires.

Employing functional magnetic resonance imaging (fMRI), a kind of neural imager which indicates which areas of the brain are more active, researchers have been able to map the regions that get more blood.

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

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

A gag activates not just the areas of the brain in charge of hearing and understanding language, but also neural regions involved in both preparation and initiating movement and those linked to vision and memory.

Put all of this together, and people listening to a joke have a complex series of neural reactions that underpin the amusement we hear.

The Infectious Nature of Chuckles

Researchers discovered that when a funny phrase is combined with laughter there is a stronger reaction in the mind than the identical word when followed by a non-emotional sound.

"This was in parts of the mind that you would employ to contort your face into a grin or a chuckle," the professor says.

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

Amusement, according to the professor, can be infectious.

So what does this mean for the chuckles found at a holiday gathering?

"You laugh harder when you know people," she notes, "and you laugh further when you like them or care for them."

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

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

The Quest for the Perfect Festive Pun

Is it possible to find the perfect joke?

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

Years ago, a psychologist established a research search for the planet's funniest joke.

More than tens of thousands of jokes submitted, with ratings lodged by hundreds of thousands of people globally, he has a clearer idea than many as to what succeeds and what does not.

The perfect Christmas cracker joke must be short, he says.

"They must also need to be bad gags, jokes that make us groan," he continues.

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

"The reason is that if nobody finds it funny – it's the gag's shortcoming, not yours.

"What's interesting about the holiday cracker puns is that none of us considers them funny.

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