A joint Canadian-Chinese study indicates that children learn to tell social lies around age four:
They asked a group of preschool children ages 3 to 6 to rate drawings by children and adults they knew, as well as strangers. The preschoolers judged the artwork both when the artist was present, and when he or she was absent. The three-year-olds were completely honest, and remained consistent in their ratings; it didn't matter who drew it, or whether the person was in the room. Five- and six-year-olds gave more flattering ratings when the artist was in front of them. They flattered both strangers and those they knew (although familiar people got a higher dose of praise). Among the four-year-olds, half the group displayed flattery while the other half did not. This supports the idea that age four is a key transitional period in children's social understanding of the world.
Lee suggests adults flatter for two reasons. It can be to show gratitude for some positive action in the past. As well, when they're meeting someone for first time – someone who may turn out to be important for their advancement down the road – flattery is also used as an investment for future favourable treatment from the person. "We don't know which the child is doing," says Lee. However, the fact that the older children flattered strangers as well as familiar people suggests "they are thinking ahead, they are making these little social investments for future benefits."
(via Collision Detection)
See also: Babies lie!
(Image: Kids painting, a Creative Commons Attribution photo from BarelyFitz's Flickr stream)