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

This one-liner is met by groans that echo through a storage facility in London.

This describes a humor-evaluation session with a firm that makes supplies for social events. Its repertoire includes Christmas crackers.

The company's founder grins, nearly sheepishly at the gag. But the joke has been selected and will feature in future crackers.

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

The key to a good Christmas cracker joke is not the same as a good gag per se. It is all about the setting - in this instance, the shared amusement of the holiday meal with grandparents, kids and possibly neighbours.

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

The Science Behind Communal Amusement

Gathering to enjoy communal amusement is not only nothing new, experts argue, it is probably to be pre-human.

"Therefore when you are laughing with others at the Christmas table you are engaging in what's very likely a truly ancient mammal social sound," says a professor.

Communal amusement, she explains, helps make and maintain social bonds between people.

Researchers have found that a lack of such social exchanges can seriously harm mental and physical well-being.

"Those you talk to, and laugh with, it leads to enhanced levels of 'happy chemical' release," the professor continues.

Endorphins are the body's "feel-good compounds" and are released both to reduce tension and discomfort and in response to pleasurable activities, such as chuckling with friends over a particularly awful festive cracker gag.

"You're not just laughing at a foolish pun with a holiday cracker," she says. "You are actually doing a lot of the truly vital task of building, preserving the social bonds you have with the people you care about."

What Happens Inside the Brain?

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

An awful lot occurs in reaction to comedy, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which indicates which parts of the brain are more active, scientists have been able to chart the regions that receive more blood flow.

Testing involves scanning the minds of volunteer subjects and then exposing them to a collection of humorous words, paired with either a non-emotional sound, or recorded chuckles.

"During the study we got a really fascinating pattern of neural activity," says the neuroscientist.

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

Put these elements together, and people listening to a joke have a complex series of neural responses that support the amusement we hear.

The Infectious Power of Chuckles

Researchers discovered that when a funny word is paired with laughter there is a stronger reaction in the brain than the same phrase when followed by a neutral sound.

"This was in areas of the mind that you would use to contort your face into a smile or a chuckle," the professor says.

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

Amusement, according to the professor, can be infectious.

So what does this mean for the laughter found around a holiday gathering?

"People laugh harder when you know others," she notes, "and laughter increases further 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 likely to be caused not by the joke itself, but from the reaction to it.

"The laughter is key. The joke is the terrible holiday cracker joke, and it's just a pretext to chuckle as a group."

The Search for the Ideal Cracker Joke

Is it possible to discover the ultimate joke?

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

Years ago, a psychologist set up a scientific project for the world's most humorous joke.

Over tens of thousands of gags submitted, with ratings lodged by 350,000 people around the world, he has a better understanding than most as to what works and what fails.

The perfect Christmas cracker pun needs to be brief, he explains.

"But they also be bad gags, puns that cause us to moan," he adds.

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

"The reason is that if nobody finds it funny – it's the joke's fault, not your own.

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

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