The bad news is: Your two-year-old may be a liar. The good news is: If she is, it's a sign of advanced cognitive skills.

Although previously, the youngest age at which children were known to lie was 3½, in an experiment by Brock University psychologist Angela Evans, lies were told by:

25 per cent of two-year-olds.

50 per cent of three-year-olds.

80 per cent of four-year-olds.

Evans published the results of the study in the journal Developmental Psychology in January in an article co-authored by University of Toronto researcher Kang Lee.

She said the most interesting finding of the study is that lie-telling appeared to be linked to brain development.

The children that didn't lie aren't necessary more "morally advanced," she told CBC Radio's Quirks & Quarks in an interview that airs Saturday.

In fact, she said, "It's that those children who are telling the lies are slightly more cognitively advanced."

Evans said she became interested in learning more about children's ability to lie, what they do and don't understand about truth and lies, and methods for promoting honesty among children, because that information is relevant to children's involvement in the legal system, for example, as witnesses.

'Really tempting' situation

For her experiment, Evans placed 41 two-year-olds and 24 three-year-olds in a "really tempting situation," in which an adult asks a child to guess the identity of a toy based on the noise it makes, for example, a quacking duck. At a certain point in the experiment, the adult leaves the room, telling the child not to peek at the next toy. A hidden camera records whether the child peeks. The adult then returns and asks the child, "Did you peek at the toy?"

While even two-year-olds appear to understand that they aren't telling the truth, they aren't deliberately trying to mislead anyone, Evans said.

"It's just that they wish they hadn't done that and so they say that they hadn't."

In addition to testing how likely children were to lie, Evans also tested the children for the brain skills needed to be able to tell a lie.

"When you think about telling a lie, you have to think about what happened, you have to prevent yourself from saying what actually happened, you have to provide an alternative response," she said.

Lie-telling was linked to better performance on a task that requires them to prevent themselves from giving the "obvious" response. In that task, they were asked to say "night" when viewing the picture of a sun and "day" when viewing a picture of a moon.

Evans notes that even though children appear able to lie at a much earlier age than previously believed, younger children tend to give away the fact that they lied if asked follow-up questions, such as the identity of the toy.

She said that gives parents the opportunity to talk about what is right or wrong.

Evans also offered one other reassuring reminder to parents.

"We all tell lies," she said. "So it's not necessarily an awful thing."