When imitating novel actions, typically developing preschoolers often copy components of the demonstration that are unrelated to the modeled action's goal, a phenomenon known as 'overimitation'. According to the social motivation account, overimitation fulfills social affiliation motives (i.e., the imitator's drive to experience social connectedness with the demonstrator and the social context). Conversely, according to the social-cognitive account, overimitation reflects overattribution of causal relevance (i.e., the imitator's failure to appreciate that some components of the demonstration are not relevant to the action's outcome). Autism Spectrum Disorder (ASD) and William syndrome (WS) are characterized by reduced and enhanced spontaneous social motivation, respectively, as well as similar impairments in social-cognition, thus providing helpful test cases to understand the nature of overimitation. Using a novel eye-tracking paradigm, we examined overimitation in 31 preschoolers with ASD, 18 age- and IQ-matched peers with WS, and 19 age-matched typically developing children. We found that children with WS and typically developing children were more likely to overimitate, and to increase their attention to the model's face during demonstration of causally irrelevant actions, compared to those with ASD. These findings will be discussed in the context of support for the social-motivational account of overimitation.