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

This joke is greeted with moans that resonate through a warehouse in the capital.

We're at a joke-testing meeting with a company that produces supplies for social events. Its catalogue includes Christmas crackers.

The firm's founder smiles, nearly sheepishly at the joke. But the pun has made the cut and will feature in future crackers.

"The success is gauged by the joke by the number of groans and the intensity of the groans at the table," the founder explains.

The key to a great holiday cracker joke is not the same as a good gag in itself. It is entirely about the setting - in this case, the shared laughter of the Christmas dinner table with elders, kids and potentially friends.

"You want the gag to be a thing that unites the eight-year-old in harmony with the 80-year-old," she adds.

The Neuroscience Behind Shared Amusement

Coming together to enjoy communal amusement is not only nothing new, scientists argue, it is likely to be pre-human.

"So when you are laughing with others at the Christmas dinner you are engaging in what's very likely a truly primordial mammal play vocalisation," explains a neuroscience expert.

Shared laughter, she explains, aids in make and maintain social connections between individuals.

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

"Those you talk to, and share laughter with, it results in enhanced levels of endorphin uptake," the professor adds.

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

"You're not just chuckling at a silly joke with a holiday cracker," she states. "You are in fact doing a lot of the truly vital task of making, maintaining the connections you have with those you care about."

What Happens In the Brain?

But what is actually happening within the mind when we hear a gag?

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

Using brain scanning technology, a kind of neural imager which shows which parts of the brain are more active, scientists have been able to map the regions that get more blood.

The research involves scanning the brains of volunteer subjects and then exposing them to a database of funny phrases, accompanied by either a neutral sound, or recorded laughter.

"In the scanner we got a very fascinating activation pattern of neural activity," notes the neuroscientist.

A gag activates not just the areas of the mind in charge of hearing and interpreting speech, but also brain regions associated with both planning and starting movement and those involved in vision and recall.

Combine all of this together, and people hearing a pun have a complex set of neural responses that support the amusement we hear.

The Contagious Power of Chuckles

Researchers discovered that when a funny word is paired with chuckles there is a greater reaction in the mind than the same word when accompanied by a non-emotional sound.

"This was in areas of the mind that you would employ to contort your expression into a grin or a laugh," she explains.

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

Amusement, says the professor, can be contagious.

So what does this imply for the chuckles heard at a holiday gathering?

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

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

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

The Quest for the Perfect Cracker Joke

Is it possible to discover the perfect gag?

Likely not, but that has not prevented researchers from attempting to.

Years ago, a psychologist established a scientific search for the planet's most humorous gag.

More than 40,000 jokes later, with scores lodged by 350,000 people around the world, he has a clearer understanding than most as to what succeeds and what fails.

The perfect festive cracker pun needs to be short, he says.

"They must also be poor jokes, puns that make us moan," he continues.

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

"The reason is that if nobody laughs – it's the joke's shortcoming, not your own.

"What's interesting about the Christmas cracker puns is that none of us considers them humorous.

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