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"How much did Santa's sled cost? Nothing, it was on the house."

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

We're at a humor-evaluation session with a company that makes products for social events. Its catalogue includes festive crackers.

The company's founder smiles, nearly apologetically at the gag. But the joke has made the cut and will feature in upcoming crackers.

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

The key to a good Christmas cracker joke is not the identical as a stand-up joke per se. It is entirely about the setting - in this case, the communal laughter of the holiday meal with elders, children and potentially neighbours.

"You want the gag to be something that unites the child in harmony with the grandparent," she states.

The Neuroscience Behind Shared Amusement

Gathering to enjoy communal amusement is not only nothing new, scientists argue, it is likely to be older than humanity.

"Therefore when you are laughing with others around the Christmas table you are dropping into what's very likely a really primordial mammal play sound," explains a professor.

Shared laughter, she explains, helps forge and strengthen social bonds between individuals.

Scientists have discovered that a lack of such social exchanges can significantly damage mental and physical health.

"The people you talk to, and laugh with, it leads to enhanced levels of endorphin release," she continues.

These natural chemicals are the body's "happy chemicals" and are produced both to alleviate stress and pain and in response to enjoyable activities, such as chuckling with loved ones over a truly awful Christmas cracker gag.

"You're not just chuckling at a silly joke with a Christmas cracker," she says. "You are actually doing a lot of the truly vital task of building, preserving the connections you have with the people you care about."

Which Occurs Inside the Brain?

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

A tremendous amount occurs in reaction to comedy, it turns out.

Employing brain scanning technology, a kind of neural imager which shows which areas of the brain are more active, researchers have been able to map the regions that receive more blood.

The research involves imaging the brains of healthy participants and then exposing them to a collection of funny phrases, accompanied by either a non-emotional sound, or recorded chuckles.

"In the scanner we got a very fascinating activation pattern of activation," says the professor.

A joke stimulates not just the parts of the brain in charge of hearing and interpreting speech, but also brain areas associated with both preparation and starting motion and those involved in vision and recall.

Put these elements together, and people hearing a pun have a sophisticated set of brain reactions that underpin the amusement we experience.

The Infectious Nature of Laughter

Researchers discovered that when a funny word is paired with laughter there is a greater response in the brain than the identical phrase when followed by a non-emotional sound.

"This activation occurred in areas of the brain that you would use to contort your face into a grin or a laugh," she says.

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

Laughter, according to the professor, can be infectious.

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

"People laugh more when you are familiar with others," she notes, "and you laugh further when you are fond of them or care for them."

When it comes to festive cracker jokes, she explains, the positive effect is more likely to be triggered not by the gag in itself, but from the response to it.

"It's the laughter. The joke is the dreadful Christmas cracker joke, and it's just a pretext to laugh together."

The Quest for the Ideal Cracker Joke

Will we ever discover the perfect gag?

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

In 2001, a psychologist set up a research project for the world's most humorous gag.

More than tens of thousands of gags submitted, with ratings provided by hundreds of thousands of participants around the world, he has a better idea than most as to what succeeds and what does not.

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

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

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

"The reason is that if nobody finds it funny – it's the joke'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 table and I believe it's wonderful."