People have two integrative mental systems in place to process information about the world. One is fast and automatic, relying on emotions, intuitions, and heuristics (“System 1”); the other is slow, effortful, and deliberate, relying on conscious reflection and rational calculations (“System 2”). In the present chapter, we argue that both mental systems uniquely contribute to conspiracy beliefs. Evidence suggests that belief in conspiracy theories largely originates from System 1 processes, as underscored by research findings relating conspiracy beliefs to intuition, decreased analytic thinking, and anxious uncertainty. These insights do not preclude a role for System 2 processes in conspiracy theories, however. Inspired by social intuitionist models of morality, we suggest that once formed, people justify conspiracy theories through a deliberate process in which they selectively search for evidence to support their suspicions. We conclude that conspiracy theories originate through System 1 processes, but people justify and maintain them through System 2 processes that involve motivated reasoning.