 Participation in this study will last 14 weeks. Both healthy people and people with SAD will be recruited to participate. All participants will complete similar study visits at entry, within 2 weeks of entry, and 12 weeks after that. The first visit, which will occur at study entry, will include screening questionnaires, an interview with research staff, a medical screening, a urine test, and collection of saliva samples for genotyping. The second visit and the last visit, which will be separated by 12 weeks, will involve MRI scans and behavioral tasks to be conducted inside and outside the MRI scanner. Over the 12 weeks between MRI scanning sessions, participants with SAD will take sertraline, a common SSRI, on a daily basis. They will also attend five additional visits during this time to complete assessments of their symptoms. These visits will occur 1, 2, 4, 8, and 12 weeks after starting sertraline treatment. Participants with SAD will therefore be completing a symptom assessment, MRI scans, and behavioral tasks all on the final visit, 12 weeks after the second visit. 