PURPOSE OF REVIEW:In this review, we summarize current knowledge and hypotheses on the nature of social abnormalities in autism spectrum disorder (ASD) and Williams syndrome (WS). RECENT FINDINGS:Social phenotypes in ASD and WS appear to reflect analogous disruptions in social cognition, and distinct patterns of social motivation, which appears to be reduced in ASD and enhanced in WS. These abnormalities likely originate from heterogeneous vulnerabilities that disrupt the interplay between domain-general and social domain-specific cognitive and motivational processes during early development. Causal pathways remain unclear. Advances and research gaps in our understanding of the social phenotypes in ASD and WS highlight the importance of (1) parsing the construct of sociability, (2) adopting a developmental perspective, (3) including samples that are representative of the spectrum of severity within ASD and WS in neuroscientific research, and (4) adopting transdiagnostic treatment approaches to target shared areas of impairment across diagnostic boundaries.