When children develop the ability to deceive others is somewhat controversial. Although some researchers believe that children are capable of a type of deception by age 3, most believe that deliberate “lying” does not emerge until children are 4 to 5 years old. To lie, a child must attempt to mislead another into believing something that is false. But several experiments have shown that children under age 4 don’t realize that people can have false or wrong beliefs about something. Between ages 4 and 5, children begin to understand that people can hold false beliefs about things in the world. It’s around this time that children become more aware that they can be deceived as well.
Peskin, as cited by Perner, J. (1991). Understanding the representational mind. Cambridge, MA: MIT Press.