Grounded in the biopsychosocial models of expressive language in autism spectrum disorder (ASD) and fragile X syndrome (FXS), this dissertation included two novel studies that examined the role of biological (i.e., physiology), psychological (i.e., autistic traits, anxiety symptoms, and child affect) and social (i.e., communication partner familiarity) factors that may impact expressive language in boys with FXS and autistic boys. Study 1 compared three expressive language measures, including total number of utterances, mean length of utterance in morphemes (MLUm; grammatical complexity), and number of different words (NDW; lexical diversity), during 10-minute conversation language sampling contexts with familiar versus unfamiliar communication partners. Study 1 also investigated the extent to which autistic traits and anxiety symptoms correlated with expressive language in the two contexts. Boys with FXS did not show expressive language differences between familiar and unfamiliar interactions for any of the three expressive language variables. In contrast, autistic boys produced greater MLUms and NDWs during conversations with unfamiliar versus familiar partners. Between-group analyses revealed that autistic boys and boys with FXS performed similarly during the familiar interaction, but autistic boys produced a significantly higher MLUm compared to boys with FXS during the unfamiliar interaction. Autistic traits moreso than anxiety symptoms correlated with expressive language performance in FXS. Both autistic traits and anxiety symptoms were associated with expressive language in ASD. The second study examined differences in physiological arousal measured via electrodermal activity (EDA) and child positive and negative affect between familiar and unfamiliar interactions, as well as associations between EDA and expressive language. Both groups showed decreased levels of positive affect in the unfamiliar compared to the familiar interaction. The autistic boys also showed dampened EDA in the unfamiliar compared to the familiar context, although this was not the case in FXS. EDA was not associated with expressive language in the boys with FXS, although in the autistic participants, EDA was positively associated with all three measures of expressive language during the familiar interaction. This set of studies were the first to examine associations between EDA, anxiety, and expressive language across conversation contexts varying in partner familiarity. The differential associations across contexts and groups suggest that different intrinsic characteristics may contribute to expressive language skills within different contexts in ASD and FXS. This dissertation lays the foundation for future work to continue examining underlying mechanisms and psychological processes that contribute to successful communication in ASD and FXS. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]