Christmas is sparkling out of every corner and parents across the country are surreptitiously peeking at letters to Santa and pulling dusty Christmas stockings out of storage. The lengths that parents go to to perpetuate the myth of Santa are an indication of how surprisingly sceptical children are when it comes to imaginary worlds.

As a society, we tell children all sorts of wonderful and fantastical things but for the most part even preschoolers are very good at knowing the difference between reality and pretence. By 3-years of age children easily distinguish pretend actions from real ones, imaginings from reality and toys from the objects they represent. By 5-6, most children in the West understand that germs and oxygen exist, even though they can’t see them, but confidently deny the existence of mermaids, monsters and Harry Potter.

In fact, young children may be less prone to being gullible. Studies of children in Madagascar and South Africa show that they endorse significantly less supernatural beliefs than their parents. And 4-year-olds are more likely to be general sceptics than 6-year-olds, who are often more credulous.

If children are so sceptical, why then does belief in Santa persist for so long? Jacqueline Woolley and colleagues at the University of Texas in Austin showed that belief in Santa in America did not decline until around 7-years of age and at 9-years a third of children still claimed that he was real.

In fact the strength of belief seems to increase – more 5-year-olds said that Santa was real than 3-year-olds. Those that didn’t believe in Santa also tended not to believe in other characters they had never seen, such as the rubbish collector.

And its not simply because 5-year-olds have had more exposure to the idea. Presented with an entirely new fantastical character – the Candy Witch –more 5-year-olds again stated that she was real than 3-year-olds. This surprising set of findings suggests that reality judgments are not simply a matter of becoming less gullible with age.

Believing what they are told is a fundamental part of learning for humans. Much of the knowledge needed to function in complex societies could not be accumulated through first-hand experience alone. However, even very young children appear to be using relatively sophisticated filters to decide what to believe and what not to.

By 3-4 years, children are discriminating about whom they listen to. They tend to trust people who are familiar or appear to have more expertise. They keep track of how accurate a speaker has been in the past and weigh up why that character might have made mistakes.

Preschoolers also attend to the context that characters are presented in. They tend not to believe in characters that perform impossible acts or exist in worlds where impossible things happen, such as mermaids, monsters and Harry Potter. However they do believe in characters that are discussed in terms of their physical effects on the real world – ‘germs make you sick’ or ‘the tooth fairy will leave you money’.

And even young children are looking for evidence to support their beliefs. In the Candy Witch study, 5-year-olds whose Halloween sweets had been replaced with a toy in the morning were much more likely to subsequently say the Candy Witch was real than those who has simply been told about her.

Despite all these truth-seeking filters, circumstances conspire against children when it comes to Santa. They are told about him by those they trust who talk about him in the context of the effects he has on the real world – ‘Santa has eaten all the biscuits’ or ‘Santa has left you presents’. And there is physical evidence to prove his existence – the biscuits really are gone and the presents really are there.

Many parents go to great lengths to convince even the sceptical child – wrapping presents in different paper, writing notes in different handwriting, and leaving snowy footprints on the rug. And as if this wasn’t convincing enough, society as a whole endorses the myth – Santa’s grottos have popped up in shopping centres across the country, families can track Santa’s progress on Norad’s website and many weather channels on Christmas Eve will plot his route and predict what meteorological hurdles he will face.

There is some debate as to whether parents should perpetuate the myth of Santa to their children. Arguments against include invalidating children’s trust, using Santa as a behavioural control and setting them up for disappointment. However, proponents point out that when children do work out that Santa is not real they tend to be delighted with their own cleverness rather than devastatingly disappointed. As cons go, the ones that come with magic and presents probably won’t be held against you for long.