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"What was the price did Santa's sleigh cost? Nothing, it was on the house."

This quip is met by moans that echo through a warehouse in London.

This describes a joke-testing meeting with a company that produces products for gatherings. Its catalogue features Christmas crackers.

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

"You measure the gag by the number of moans and the loudness of the groans at the table," she says.

The key to a great Christmas cracker joke is not the same as a good gag in itself. It is entirely about the setting - in this instance, the shared amusement of the holiday meal with grandparents, children and potentially neighbours.

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

The Neuroscience Behind Shared Laughter

Coming together to experience shared laughter is not only nothing new, experts argue, it is likely to be older than humanity.

"So when you are chuckling with people at the Christmas dinner you are dropping into what's almost certainly a really primordial mammal play sound," says a professor.

Shared amusement, she explains, aids in forge and strengthen social bonds between individuals.

Researchers have found that a lack of these interactions can significantly harm mental and physical well-being.

"Those you converse with, and share laughter with, it results in increased levels of endorphin release," the professor adds.

These natural chemicals are the brain's "happy chemicals" and are released both to reduce stress and pain and in reaction to pleasurable activities, such as chuckling with friends over a truly terrible festive cracker joke.

"It's not simply chuckling at a silly joke with a holiday cracker," she says. "You are in fact performing a lot of the really vital work of making, maintaining the social bonds you have with the people you care about."

What Happens Inside the Brain?

But what is truly happening inside the brain when we hear a joke?

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

Employing functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which areas of the brain are working harder, researchers have been able to map the regions that get more blood flow.

The research involves scanning the minds of healthy subjects and then subjecting them to a database of humorous phrases, accompanied by either a non-emotional sound, or recorded laughter.

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

A joke stimulates not just the parts of the brain in charge of auditory processing and interpreting language, but also neural regions associated with both planning and initiating motion and those involved in sight and recall.

Put all of this as a whole, and individuals hearing a joke have a complex set of neural reactions that underpin the amusement we experience.

The Infectious Nature of Chuckles

Researchers discovered that when a humorous word is combined with laughter there is a stronger reaction in the mind than the identical word when accompanied by a neutral sound.

"This was in areas of the mind that you would use to contort your expression into a grin or a chuckle," she says.

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

Amusement, according to the expert, can be infectious.

So what does this imply for the laughter found at a Christmas gathering?

"You laugh more when you are familiar with others," she says, "and laughter increases further when you are fond of them or love them."

When it comes to festive cracker jokes, she says, the feel-good effect is more probable to be triggered not by the joke in itself, but from the response to it.

"It's the laughter. The gag is the terrible holiday cracker pun, and it's just a pretext to chuckle together."

The Search for the Perfect Festive Pun

Is it possible to discover the perfect joke?

Probably not, but that has not prevented experts from trying to.

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

Over tens of thousands of gags later, with ratings lodged by hundreds of thousands of participants globally, he has a better understanding than most as to what succeeds and what fails.

The ideal festive cracker joke must be short, he says.

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

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

"This is because if no-one finds it funny – it's the joke's shortcoming, not your own.

"What's interesting about the holiday cracker jokes is that none of us find them humorous.

"It creates a shared experience at the gathering and I believe it's lovely."