The author presented 60 9th- and 12th-graders with hypothetical arguments that contained logical fallacies. Arguments were either consistent or inconsistent with participants' theories. Participants rated the quality and truth of each argument, identified perceived strengths and weaknesses in the arguments, and verbally described hypothetical experiments that could lead to evidence falsifying the claims made in the arguments. Results indicated that intellectual ability, particularly verbal ability, was the best predictor of each index of everyday reasoning. However, neither the ability measures nor age were related to biases in everyday reasoning. Hierarchical multiple regression analyses showed that, for each reasoning variable, adolescents' personal theories accounted for the most variance in reasoning biases. These findings are discussed in terms of the roles that intellectual ability and theory-driven motivation play in everyday reasoning and self-serving adolescent reasoning.