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"How much did Santa's sled cost? Zero, it was on the house."

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

This describes a humor-evaluation session with a firm that makes products for social events. Its repertoire features festive crackers.

The firm's owner smiles, almost sheepishly at the gag. But the joke has made the cut and will appear in upcoming crackers.

"You measure the gag by the number of groans and the intensity of the groans around the table," she explains.

The secret to a great holiday cracker joke is not the same as a good gag in itself. It is all about the context - in this instance, the shared laughter of the holiday meal with elders, children and possibly friends.

"The goal is for the gag to be something that unites the child together with the 80-year-old," she states.

The Neuroscience Behind Communal Amusement

Gathering to experience communal laughter is not only nothing new, scientists argue, it is probably to be older than humanity.

"So when you are chuckling with others at the holiday dinner you are dropping into what's almost certainly a truly primordial mammal play vocalisation," explains a professor.

Communal laughter, she says, helps make and maintain social bonds between individuals.

Scientists have discovered that a absence of such interactions can seriously damage both psychological and bodily health.

"The people you converse with, and share laughter with, it results in increased levels of 'happy chemical' release," she adds.

Endorphins are the brain's "feel-good compounds" and are produced both to alleviate stress and pain and in response to pleasurable activities, such as chuckling with loved ones over a particularly terrible Christmas cracker joke.

"It's not simply chuckling at a silly joke with a Christmas cracker," the expert states. "You are in fact performing a lot of the really vital task of building, preserving the connections you have with the people you care about."

What Occurs In the Mind?

But what is truly happening inside the mind when we hear a gag?

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

Employing brain scanning technology, a type of brain scanner which shows which parts of the mind are working harder, researchers have been able to chart the areas that receive more blood flow.

The research entails imaging the minds of volunteer subjects and then exposing them to a collection of humorous phrases, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we got a very interesting pattern of neural activity," notes the professor.

A gag activates not just the parts of the mind responsible for hearing and understanding language, but also brain areas involved in both preparation and initiating motion and those linked to vision and memory.

Combine these elements as a whole, and people listening to a joke have a sophisticated series of brain reactions that support the laughter we hear.

The Contagious Power of Laughter

Scientists found that when a humorous word is paired with laughter there is a stronger response in the brain than the identical word when accompanied by a non-emotional sound.

"This activation occurred in parts of the brain that you would employ to contort your expression into a smile or a chuckle," she explains.

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

Amusement, according to the professor, can be contagious.

So what does this imply for the laughter heard around a holiday table?

"People laugh harder when you know others," she notes, "and laughter increases more when you like them or love them."

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

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

The Quest for the Perfect Cracker Joke

Will we ever discover the ultimate joke?

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

Years ago, a psychologist established a research project for the world's funniest gag.

More than 40,000 jokes later, with scores provided by hundreds of thousands of participants globally, he has a clearer idea than many as to what succeeds and what does not.

The ideal Christmas cracker joke must be brief, he says.

"They must also be poor jokes, jokes that cause us to groan," he adds.

The more "terrible" the joke, he says the more effective.

"This is because if no-one finds it funny – it's the gag's shortcoming, not yours.

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

"It creates a common experience around the gathering and I think it's lovely."