Liar Liar, Pants On Fire
I babysit for a child who, at 5 years old, is a master liar. This child is GOOD. Their poker face is impenetrable, and even when it is scandalously obvious that what they are saying is a lie, they will not budge. I have tried everything. I challenge their claims, I point out the fault in their logic, I blurt out "I know you are lying to me right now" and nothing works. Kid is crafty. So what do we do when we have a little politician - I mean, a little liar - in our midst?
The first thing that comes to mind is that kids lie out of necessity. In their minds, the want-take-have impulse is very strong, and they will go to any extent to get what they desire. Also, kids have no idea of what consequences mean. They live in the moment, and the outcome of being caught in a lie is not something that they consider, much less something that bothers them. This isn't out of malice, it's just that they don't have the capacity to weigh the actions with the outcomes. They want something, and they will do what they want to get it. So we shouldn't worry that our kids are diabolical, just that they don't consider the outcome when they are caught in the lie.
Now that the child is caught in the lie, what do we do? Yelling and screaming isn't going to work (does it ever?) and confronting them with the fact that "We don't lie" doesn't ring true to them because they don't understand why "we don't lie." So when the child is caught after bopping their brother or sister on the head and they insist "I didn't do it" we have to realize that the punishment has to fit the crime. Explaining that "I am making you give up your toy because you hit your sister" is one thing, but telling them "and you can't get it back until tomorrow because you lied to me about it" gives the lie consequences. Ignoring the lie reinforces that they can continue to lie, and get away with it, even though they are punished for the action.
If that doesn't work, there's always psychological warfare. I am not above telling the children in my immediate vicinity "It makes me very very sad when you lie to me" which almost always has much more weight than "it makes me very angry when you lie to me." Kids can deal with anger. Disappointment and sadness are a completely different situation. Telling a child "You make me sad when you do that" calls to their own sadness. It's an emotion that they understand, and they know that being sad is terrible, awful, and is something that they don't associate with adults. Adults get mad all the. time, but a sad adult? That's terrible! Giving that kind of emotional weight to the consequences of telling a lie will almost certainly get 100% better results than anger.
So good luck out there. There is nothing sinister about your child lying, unless it's about something extremely serious, and like I said, sometimes you have to fight fire with fire. No kid wants to make an adult sad. And if that is what it takes to get them to stop fibbing about every little thing they do, so be it.