This week in the Wall Street Journal, Shirley Wang reports that it's possible to plot the cognitive development of children along an axis of how well they can lie. As the brains of children mature, they become more skilled at fabricating stories, controlling their body language, and disguising their true knowledge of events. According to Wang, then, you shouldn't necessarily worry if your kid is getting better at lying, because all it means for sure is that his or her brain is growing more sophisticated.

Researchers have also examined why some kids lie more than others, and have found that it isn't related to better moral values or religious upbringing. Rather, it's kids with better cognitive abilities who lie more. That's because to lie you also have to keep the truth in mind, which involves multiple brain processes, such as integrating several sources of information and manipulating that information ... The ability to lie—and lie successfully—is thought to be related to development of brain regions that allow so-called 'executive functioning,' or higher order thinking and reasoning abilities. Kids who perform better on tests that involve executive functioning also lie more.

So is there a way for parents to make it clear that lying is a bad habit? Wang offers a qualified suggestion.
Some studies suggest there is no long-term effect of parenting on lying behavior, but the work of Dr. Talwar and her colleague Angela Crossman at the John Jay College at the City College of New York shows that a certain type of parenting style seems to discourage lying. They suggest parents discuss why there are rules against lying. Also, parents who point out when kids lie—and also acknowledge when children come clean—can foster more truth-telling, says Dr. Talwar.