The capacity to deceive emerges developmentally at about 3 1/2 to 4 years (girls earlier than boys). Children tend to practice newly attained milestones and at times they get a parental response that is puzzling to them and leads to "testing." A simple stern response with "disappointed in you" look, such as saying "That is not true. We need you to tell the truth."
The most common reason for somewhat older children to lie is that they think their parents will be disappointed or angry for something careless they did. It is best to be understanding about careless mistakes and know that they will do better when they gain better habits over time through daily routine expectations rather than one time requests when they may have their mind elsewhere. Punishments are not an effective was to create habits and are more likely to make dishonesty look like it may be worth a try.
Many parents ask children questions about misdeeds when they know the answers in order to catch the child in a lie. It is better to "Tell don't ask" when you know there was a misdeed and have a consequence that requires the child to make up for the deed.
The most effective way to teach honesty is to have adult models for honest behavior as well as having a community that echoes these standards such as a religious group.
