Promising to tell the truth makes 8- to 16-year-olds more honest.

Techniques commonly used to increase truth-telling in most North American jurisdiction courts include requiring witnesses to discuss the morality of truth- and lie-telling and to promise to tell the truth prior to testifying. While promising to tell the truth successfully decreases younger children's lie-telling, the influence of discussing the morality of honesty and promising to tell the truth on adolescents' statements has remained unexamined. In Experiment 1, 108 youngsters, aged 8-16 years, were left alone in the room and asked not to peek at the answers to a test. The majority of participants peeked at the test answers and then lied about their transgression. More importantly, participants were eight times more likely to change their response from a lie to the truth after promising to tell the truth. Experiment 2 confirmed that the results of Experiment 1 were not solely due to repeated questioning or the moral discussion of truth- and lie-telling. These results suggest that, while promising to tell the truth influences the truth-telling behaviors of adolescents, a moral discussion of truth and lies does not. Legal implications are discussed.