 Anxiety is a common and significant problem for people suffering from disorders of nervous system development, including fragile X syndrome. There are few validated treatments for anxiety in people with these disorders, in part because the biological basis of anxiety in neurodevelopmental disorders has not been clearly described. This study will evaluate the physiological responses of people with fragile X syndrome, Down syndrome, and the fragile X premutation a mild version of the genes that cause fragile X syndrome to a variety of sensory, emotional, and social stimuli. By analyzing the data collected for this study, researchers aim to identify physiological characteristics linked to subgroups within the disorders, demonstrate links between physiological responses and behavioral or psychiatric symptoms, and measure physiological changes in people receiving treatment for their disorders. 