Fragile X syndrome (FXS) is caused by expansion of DNA sequence in the Fragile X Mental Retardation-1 gene (FMR1) and is the leading single-gene disorder associated with a diagnosis of autism. Atypical language prosody, which is the perception of rhythm, stress, and intonation of speech, is a core feature of autism and can compromise communicative competence in daily interactions. Researchers in this study will determine the differences and overlap in prosodic profiles of boys with FXS and autism. The research will compare language prosody of boys with autism spectrum disorder (ASD) only (ASD-O), FXS with ASD (FXS-ASD), FXS only (FXS-O), and typical development (TD) to identify precise autism phenotypes that could be linked with the Fragile X Mental Retardation-1 gene (FMR1). 40 boys with ASD-O, 40 boys with FXS-ASD, 40 boys with FXS-O, and 50 TD boys (25 with similar language age [TD-LA] and 25 with similar chronological age [TD-CA]) will be assessed once over a 2-year period. The boys' language in imitated sentences and conversational speech will be examined for prosody using both objective, acoustic measures and listener judgments of phrasing, rate, and stress. In addition, researchers will assess perceived peculiarity of speech. Relationships between judgments of prosodic peculiarity and specific aspects of prosody, as well as the relationship between social cognition (theory of mind) and prosody, will be examined. As well as furthering our understanding of the association between autism and FMR1, determining the differences and overlap in prosodic profiles of boys with FXS and ASD using both rater judgments and objective measures has important clinical implications and may aid in identifying effective treatment protocols.