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

This quip is greeted with moans that echo through a storage facility in London.

We're at a humor-evaluation session with a firm that makes supplies for social events. Its catalogue includes Christmas crackers.

The company's founder grins, nearly apologetically at the gag. But the pun has been selected and will appear in upcoming crackers.

"The success is gauged by the joke by the volume of moans and the intensity of the groans at the table," the founder says.

The secret to a good Christmas cracker joke is not the identical as a good gag per se. It is all about the setting - in this instance, the communal amusement of the Christmas meal with elders, children and possibly friends.

"The goal is for the joke to be something that unites the eight-year-old in harmony with the 80-year-old," she adds.

The Neuroscience Of Communal Amusement

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

"So when you are laughing with people at the holiday table you are engaging in what's almost certainly a really ancient mammal play sound," explains a neuroscience expert.

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

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

"The people you talk to, and share laughter with, it results in increased levels of 'happy chemical' release," she continues.

Endorphins are the body's "happy chemicals" and are released both to alleviate stress and pain and in response to pleasurable activities, such as laughing with friends over a particularly terrible Christmas cracker gag.

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

What Occurs Inside the Brain?

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

An awful lot happens in response to comedy, it turns out.

Using functional magnetic resonance imaging (fMRI), a type of neural imager which indicates which areas of the brain are more active, researchers have been able to map the regions that get more blood flow.

Testing entails scanning the minds of healthy subjects and then subjecting them to a database of humorous phrases, paired with either a neutral sound, or recorded chuckles.

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

A gag activates not just the areas of the brain in charge of hearing and understanding language, but also brain areas involved in both preparation and initiating motion and those involved in vision and recall.

Combine these elements as a whole, and people listening to a joke have a sophisticated set of neural reactions that underpin the laughter we experience.

The Contagious Nature of Chuckles

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

"This activation occurred in parts of the mind that you would employ to contort your face into a grin or a chuckle," she says.

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

Amusement, says the expert, can be infectious.

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

"People laugh harder when you know others," she says, "and laughter increases further when you are fond of them or love them."

When it comes to Christmas cracker jokes, she explains, the positive effect is more probable to be caused not by the gag in itself, but from the reaction to it.

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

The Search for the Ideal Cracker Joke

Will we ever discover the perfect joke?

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

In 2001, a psychologist established a research search for the world's most humorous gag.

More than tens of thousands of jokes submitted, with scores provided by hundreds of thousands of people globally, he has a clearer idea than many as to what succeeds and what fails.

The perfect festive cracker pun must be short, he says.

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

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

"The reason is that if no-one laughs – it's the gag's shortcoming, not yours.

"The fascinating part about the holiday cracker puns is that none of us find them humorous.

"That's a shared moment at the table and I believe it's wonderful."