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

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

We're at a humor-evaluation meeting with a company that produces supplies for gatherings. Its repertoire includes Christmas crackers.

The company's founder grins, almost apologetically at the joke. But the joke has been selected and will feature in upcoming crackers.

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

The secret to a great holiday cracker joke is not the same as a stand-up gag per se. It is entirely about the setting - in this instance, the communal laughter of the Christmas meal with elders, children and possibly friends.

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

The Neuroscience Behind Communal Laughter

Gathering to enjoy communal amusement is not only ancient, experts argue, it is probably to be older than humanity.

"Therefore when you are laughing with others at the Christmas table you are dropping into what's almost certainly a truly ancient mammalian play sound," says a professor.

Communal amusement, she says, helps make and maintain social connections between people.

Scientists have discovered that a absence of such interactions can seriously harm mental and physical well-being.

"The people you talk to, and share laughter with, it results in increased levels of endorphin release," the professor adds.

These natural chemicals are the brain's "feel-good compounds" and are released both to alleviate stress and pain and in reaction to enjoyable experiences, such as chuckling with loved ones over a particularly awful festive cracker gag.

"You're not just chuckling at a foolish pun with a Christmas cracker," she says. "You are in fact performing a lot of the really important work of building, preserving the social bonds you have with the people you love."

Which Occurs Inside the Brain?

But what is actually taking place within the mind when we hear a joke?

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

Employing 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 areas that receive more blood flow.

Testing involves scanning the brains of volunteer participants and then subjecting them to a database of humorous phrases, paired with either a non-emotional sound, or recorded chuckles.

"During the study we got a really fascinating activation pattern of neural activity," notes the professor.

A gag stimulates not just the parts of the brain in charge of hearing and understanding language, but also neural regions associated with both planning and starting motion and those involved in sight and recall.

Combine all of this together, and individuals hearing a pun have a complex series of neural reactions that support the laughter we experience.

The Infectious Nature of Laughter

Scientists discovered that when a funny phrase is paired with laughter there is a stronger reaction in the brain than the same word when accompanied by a non-emotional sound.

"This was in parts of the brain that you would use to contort your expression into a smile or a laugh," she says.

It means people are not just responding to humorous jokes, they are reacting to the laughter that accompanies them.

Laughter, according to the professor, can be contagious.

So what does this imply for the laughter heard at a Christmas table?

"You laugh more when you are familiar with people," she says, "and you laugh more when you like them or care for them."

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

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

The Quest for the Ideal Cracker Joke

Is it possible to find the ultimate gag?

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

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

Over tens of thousands of jokes submitted, with ratings provided by 350,000 participants around the world, he has a better idea than many as to what works and what fails.

The ideal festive cracker pun must be brief, he explains.

"But they also need to be poor jokes, puns that cause us to groan," he continues.

The more "terrible" the gag, he says the better.

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

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

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