Social communication impairment is one of the key diagnostic features of ASD, with communication deficits being the earliest symptom reported by most parents of children with ASD (Filipek et al., 1999; Landa & Garrett-Meyer, 2006). From differences in babble (Trevarthen & Daniel, 2005; Yirmiya, et al., 2006) and gesture inventories (Landa et al., 2007; Wetherby et al., 1998) to delayed onset or atypical development of verbal language (Luyster, 2009) and social-communication deficits (Landa et al., 2007; Wetherby, 1998; Zwaigenbaum, 2005), children with ASD present with a unique profile of early communication behaviors. One aspect of early communication development that has received limited attention is communication spontaneity (CS). CS has been conceptualized as the level of environmental support required for a child to initiate a communication act (Carter, Hotchkis & Cassar, 1996). While CS has been discussed as an important aspect of communication, additional research is needed to better understand CS's nature (i.e., how CS changes over time) and relevance (i.e., relationship to other developmental domains) in typically developing children as well as children with developmental disabilities such as ASD. Given the limitations of retrospective report, prospective studies are necessary to determine if early risk signs are related to later development of ASD or broader autism phenotype (BAP) features. The purpose of the present study was to examine CS in a cohort of infants at high and low genetic risk for ASD at 12 and 24 month of age. Results indicated that CS is associated with existing measures of language and social communication. A specific component of CS which captures how a child responds to direct communication prompts (CS-SP-Direct Prompts), was found to be particularly associated with outcomes, including ADOS diagnostic group and symptom severity. In contrast, children's CS during Free Play conditions or in response to non-social prompts was not significantly related to ASD outcomes. These findings suggest that previous discussions of potential differences in CS may be better conceptualized as differences in Social Prompt Responsiveness. At 12 months of age, CS-SP-Direct Prompts differentiated ASD from NonASD children and uniquely predicted both diagnostic group and ASD symptom severity. CS-SP-Direct Prompts may also be associated with the broader autism phenotype, though differences may not become clear until 24 months of age in high risk (HR), NonASD siblings. Results from the present study suggest that structured measurement of Social Prompt Responsiveness is possible and may contribute important information regarding a child's social-communication development, particularly in monitoring infants at high genetic risk for ASD. Findings also provide support for the social salience framework of ASD and its application to early communication development. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]