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

This quip is greeted with groans that resonate through a storage facility in London.

We're at a humor-evaluation session with a firm that makes supplies for gatherings. Its repertoire features Christmas crackers.

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

"The success is gauged by the joke by the volume of moans and the loudness of the groans at the table," she says.

The key to a good holiday cracker joke is not the same as a good gag per se. It is all about the setting - in this instance, the communal laughter of the holiday meal with elders, children and possibly neighbours.

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

The Neuroscience Of Communal Laughter

Coming together to experience communal amusement is not only ancient, experts argue, it is probably to be pre-human.

"Therefore when you are chuckling with people at the holiday dinner you are dropping into what's very likely a truly primordial mammal play vocalisation," says a neuroscience expert.

Communal amusement, she says, helps forge and strengthen social bonds between individuals.

Researchers have discovered that a lack of such social exchanges can seriously harm mental and physical health.

"The people you converse with, and laugh with, it leads to increased amounts of 'happy chemical' release," the professor continues.

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

"It's not simply chuckling at a silly pun with a holiday cracker," the expert states. "You are in fact performing a lot of the really vital work of making, maintaining the connections you have with the people you love."

Which Occurs In the Mind?

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

A tremendous amount happens in response to comedy, it turns out.

Using brain scanning technology, a kind of neural imager which indicates which parts of the mind are more active, researchers have been able to chart the areas that get more blood flow.

The research involves imaging the brains of volunteer participants and then subjecting them to a database of funny words, accompanied by either a non-emotional sound, or recorded chuckles.

"During the study we observed a very fascinating pattern of activation," notes the professor.

A joke stimulates not just the parts of the mind responsible for auditory processing and understanding speech, but also neural areas involved in both planning and starting movement and those involved in vision and recall.

Combine all of this together, and people hearing a pun have a sophisticated set of neural reactions that underpin the amusement we hear.

The Contagious Power of Laughter

Scientists discovered that when a funny phrase is paired with chuckles there is a stronger response in the mind than the same phrase when followed by a non-emotional sound.

"This activation occurred in areas of the brain that you would use to move your expression into a grin or a chuckle," she says.

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

Amusement, according to the expert, can be contagious.

So what does this imply for the chuckles heard around a Christmas table?

"People laugh more when you know others," she says, "and you laugh more when you are fond of them or love them."

When it comes to Christmas cracker jokes, she says, the positive effect is more probable to be caused not by the gag itself, but from the response to it.

"The laughter is key. The joke is the terrible Christmas cracker pun, and it's just a pretext to laugh as a group."

The Quest for the Ideal Festive Pun

Will we ever discover the perfect joke?

Probably not, but that has not stopped experts from attempting to.

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

Over tens of thousands of gags submitted, with scores provided by 350,000 people globally, he has a clearer understanding than many as to what succeeds and what fails.

The perfect festive cracker joke needs to be brief, he says.

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

The increasingly "terrible" the joke, he says the better.

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

"The fascinating part about the holiday cracker jokes is that none of us considers them funny.

"It creates a shared experience around the gathering and I believe it's wonderful."