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. Copyright © 2010 John Wiley & Sons, Ltd.