Deductive reasoning is influenced by emotions and beliefs. It is unknown how factors relating to emotion and beliefs interact to affect logical reasoning. This dissertation explored the nature of emotion-belief interactions in relational reasoning using behavioral, genetic and neuroimaging techniques. In Study 1, reasoning behavior was influenced by beliefs such that participants were less accurate and slower to evaluate arguments in which the conclusion validity conflicted with beliefs, thereby replicating the belief bias effect. Moreover, belief-bias interacted with emotional content and serotonin transporter genotype; carriers of the short (S) allele of the serotonin transporter gene (5-HTTLPR) had increased emotional belief bias relative to long (L) carriers. Groups did not differ in non-emotional belief bias. In Study 2, I investigated the neural basis for this emotion-belief interaction using functional magnetic resonance imaging (fMRI). Healthy individuals with the S allele had decreased activation of right inferior frontal cortex (rIFC) and increased activity in ventromedial prefrontal cortex (vmPFC) when reasoning with emotional belief-logic conflict problems relative to L allele carriers. Anxiety was positively correlated with activation of vmPFC in emotional belief-logic conflict reasoning. These results have implications for depression and anxiety disorders, which are related to the 5-HTTLPR S allele, emotional reactivity, and deleterious beliefs.