Self-Serving Influences on Adolescents' Evaluations of Belief-Relevant Evidence

To examine the effects of motivation and intellectual ability on adolescent reasoning, 50 adolescents were presented "scientific" evidence relevant to their religious affiliations. Tests of fluid and crystallized intelligence were also administered. Belief-consistent evidence portrayed adolescents' religious affiliations favorably; inconsistent evidence portrayed these religions unfavorably; neutral evidence was irrelevant to participants' religions. Each type of evidence contained a threat to internal validity, detection of which would have discredited the evidence. Results indicated that validity threats were detected most frequently in belief-inconsistent evidence, indicating that adolescents' reasoning is systematically biased to protect and promote their preexisting beliefs. A manipulation designed to motivate adolescents toward greater accuracy improved overall performance, but did not reduce bias. Crystallized intellectual ability was linked to absolute level of reasoning and to the effectiveness of the accuracy manipulation. However, reasoning biases cut across levels of intellectual ability. The role played by depth of processing in supporting biased reasoning is discussed.