Lying is nothing unusual for small children, and a new study says it's actually an important developmental milestone.

A few months ago, my family ordered in pizza for dinner. When I noticed my son's plate had been cleared in record time, I asked him if he had really eaten the entire thing himself. "Yup!" he said proudly.

But then later that day, when I went to toss something in the kitchen trash compactor, I found a full slice wrapped in a paper plate lying on top.

"Dude, are you sure you finished your pizza?" I asked my 10-year-old again, this time presenting him with the new piece of evidence. But he just looked at it and shrugged. "Mommy, I swear," he said, all innocent. "It's not mine."

At the time, I was uncomfortably floored by my kid's blatant deception. But then I read about a study that says lying is nothing unusal for small children. And in fact, learning to lie is apparently considered an important developmental milestone.

Wait, what?

According to research led by Kang Lee, a psychology professor at the University of Toronto, lying begins early in precocious kids. Among verbal 2-year-olds, 30 percent give it a go at some point. By age 3, about half are trying it regualrly. At age 4, that number rises to 80 percent. And by the time kids are 5 to 7, nearly all of them are lying.

What's more, Dr. Lee said that younger children who bend the truth have a cognitive advantage over the ones who don't. "Lying requires two ingredients," he explained. "Children need to understand what's in someone else's mind—to know what they know and what they don't know. We call this ability theory of mind. The children who are better at theory of mind are also better at lying."

The second requirement is executive function, otherwise known as the power to plan ahead and curb unwanted actions. "The 30 percent of the under-3s who can lie have higher executive function abilities," Dr. Lee said, "specifically the ability to inhibit the urge to tell the truth and to switch to lying." Such cognitive sophistication, he added, means that these early liars will be more successful in school and in their dealings with other kids on the playground.

Dr. Lee conducted theory-of-mind experiments (a popular tool for helping children on the autistic spectrum and those with behavioral problems) on a group of 58 preschoolers in China, dividing them into two groups after testing them for such things as intelligence, lying ability, and executive function.

Half of the children received six sessions of theory-of-mind training and the other half received an equal number of sessions devoted to teaching number and spatial problem-solving skills. Researchers found that the children in the theory-of-mind group had not only become better liars but also were significantly better at lies than the control-group children were.

"The first occasion of your child telling a lie is not an occasion to be alarmed but an occasion for celebration," Kang told the Wall Street Journal.

Well, that's a relief!