How should you answer when your child asks 'is Santa real?'

But there's one big, fat, red and white lie many of us are happy to tell.

"You've got your parents on one side who want to preserve the magic of Christmas by telling kids Santa is real versus those who kind of struggle with the morality of lying," University of Technology Sydney clinical psychologist Dr Rachael Murrihy told ACM.

"And I think the important thing to take away is there is no evidence that children are harmed in either case."

At the heart of the argument to tell children the truth about Santa from the get-go, is that by lying to a child, trust is being eroded in the relationship and by learning a parent has lied, a child may learn to lie themselves.

But Dr Murrihy said this shouldn't concern parents.

"We all do a bit of pro-social lying in interpersonal situations, that's normal, but I think if parents are generally telling the truth about most issues then one exception that's Santa is not going to cause your child to be a dishonest child," she said.

The argument in favor of the lie, on the other hand, is twofold.

Firstly, University of Adelaide emotions researcher and PhD candidate Nathan Jones said, a child's belief in Santa Claus can be a source of excitement, joy and awe.

"When kids have that sense of awe and wonder, that can really foster imagination and playfulness and that can lead to all sorts behaviors that are quite adaptive," he said.

"That could be writing a letter to Santa, for instance, which is quite creative and fosters a sense of wonder and imagination, or making cookies and putting them out on Christmas Eve.

"They're all positive emotions, and they're all things that are adaptive that could come from a belief in Santa."

And then there's the prompt for children to behave because 'Santa is watching'.

Although, Mr Jones said, this can cause anxiety in some children.

"Every child is so different. It may be if you've got more of a sensitive child, when they hear that 'presents and gifts are only given to the good kids,' some kids may respond to that in a way where it's more of a challenge to be good and that's fine," he said.

"For sensitive kids, it can actually cause a lot of anxiety ... confusion could be another one; 'why this is all going on and why my behavior is now being held against me?'"

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So, what is the expert advice for when you're asked the big question: "Is Santa real?"

UTS's Dr Murrihy said to consider the context of your child's question.

"If they're coming to you and saying, 'my friend at school has said it's not true. That's not right, is it?' You know, if they're asking in disbelief, then you may want to continue telling them Santa is real," she said.

"But if they come to you and say, 'look, this doesn't make any sense. A fat man can't fit down the chimney' ... then you might want to tell them the truth."