In Chapter 4 of NurtureShock, Bronson and Merryman make some fascinating observations about children's attitudes and behaviours around lying.
For a start, adults are not very good at telling whether kids are lying or not, relying too often on cues like gender and extroversion (p. 75). Even teachers score only 60% accuracy in identifying lies under controlled conditions.
Parents generally believe their kids won't lie, and see lying as a negative behaviour – usually one to be strongly discouraged. But studies show (p. 80) that at three or four most children become active and proficient liers. Interestingly, the kids who best know the difference between the truth and a lie are the ones most likely to lie. Lying is also a complex cognitive task (p. 82). Effective fabrication requires a good understanding of the situation, of people's behaviours, of what may or may not sound credible.
The problem arises when you think about the range of behaviours that fall under the umbrella of lying. When asked whether they took the money out of mum's purse, parents believe that truth is the best policy. However, what about when grandma gives you a pair of socks for Christmas and asks you how you like them? Dishonesty can become a valued social behaviour in particular situations.
The 'black and white' approach to lying is problematic – it also gets us into trouble when we go back on promises we have made, or sugar-coat situations… behaviours that we see as part of normal behaviour, but a young child will see clearly as 'a lie'. What kids need to start to understand to function well socially (including within the family) is the motivations that underlie 'dishonest' behaviour – and hence why 'lying' is wrong. When asked why lying is wrong, most five year olds will say it's because you get punished for it – good old reward and punishment removing the intrinsic motivator not to lie and replacing it with an extrinsic one. At that age, 38% of kids actually believe that swearing is lying. Why? Because they are both things you can say that you get punished for! (p.84)
Threatening punishment for lying focuses the child on the impact that lies can have on them. They won't lie less – they will get better at it to avoid punishment. Bronson and Merryman's advice is to leverage children's desire to please parents by demonstrating that truth-telling is behaviour that will make mum and dad happy – focus less on lying being bad, and more on honesty being good (p.86).
At the end of the day, we spend a lot of time admonishing kids for lying when many of us do it every day because that's how we get through life. Understanding the effect your lies (and behaviours more generally) will have on others will be the most pragmatic grounding for truth-telling. This is a very teleological approach – justifying the end by the means – but it is the one that most adults claim to operate by. Like so many other social skills, the thing it relies on most heavily is empathy; modelling the thoughts and behaviours of others will inform our decisions when it comes to telling the truth… or not.