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

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

This describes a humor-evaluation session with a firm that produces supplies for social events. Its catalogue features festive crackers.

The company's founder smiles, almost sheepishly at the gag. But the pun has been selected and will feature in upcoming crackers.

"The success is gauged by the gag by the number of groans and the intensity of the groans at the table," she says.

The secret to a great Christmas cracker joke is not the identical as a stand-up gag in itself. It is all about the setting - in this case, the shared laughter of the Christmas dinner table with elders, children and potentially neighbours.

"You want the joke to be something that brings the eight-year-old in harmony with the 80-year-old," she states.

The Neuroscience Behind Communal Laughter

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

"Therefore when you are laughing with others at the holiday dinner you are engaging in what's very likely a really primordial mammal social sound," explains a professor.

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

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

"The people you converse with, and share laughter with, it results in enhanced levels of endorphin uptake," the professor continues.

Endorphins are the body's "feel-good compounds" and are released both to reduce tension and discomfort and in reaction to enjoyable activities, such as laughing with loved ones over a particularly terrible Christmas cracker gag.

"It's not simply chuckling at a silly joke with a Christmas cracker," she states. "You are actually performing a lot of the truly important task of making, maintaining the social bonds you have with those you love."

Which Occurs Inside the Mind?

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

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

Employing functional magnetic resonance imaging (fMRI), a type of brain scanner which indicates which areas of the brain are working harder, scientists have been able to chart the regions that get more blood flow.

The research involves imaging the minds of volunteer subjects and then subjecting them to a database of funny phrases, paired with either a non-emotional sound, or recorded laughter.

"During the study we observed a really interesting pattern of activation," says the professor.

A gag activates not just the areas of the mind in charge of auditory processing and understanding speech, but also brain areas involved in both planning and initiating motion and those involved in vision and recall.

Put all of this as a whole, and people listening to a joke have a sophisticated series of neural responses that support the amusement we hear.

The Infectious Nature of Chuckles

Researchers found that when a humorous word is paired with chuckles there is a stronger response in the brain than the same word when followed by a non-emotional sound.

"This was in parts of the brain that you would use to move your face into a grin or a chuckle," the professor says.

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

Amusement, says the expert, can be contagious.

So what does this mean for the laughter found at a Christmas table?

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

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

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

The Search for the Perfect Cracker Joke

Is it possible to discover the perfect gag?

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

Years ago, a psychologist set up a research project for the planet's funniest joke.

More than 40,000 gags later, with scores lodged by hundreds of thousands of people around the world, he has a better idea than many as to what works and what does not.

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

"But they also be poor gags, jokes that make us moan," he adds.

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

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

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

"It creates a shared moment around the gathering and I believe it's lovely."