This study examines how spontaneous interests in people and in objects relate to joint engagement in typically developing toddlers and young children with autism or Down syndrome. Ratings of interests were made repeatedly during intermissions in a laboratory-based protocol focused on caregiver-child interactions. Interests were moderated by diagnosis and relatively stable across intermissions. In autism, interest in people tended to be low and to decline rapidly, and the balance of interests favored familiar objects over people. Lower interest in people and in unfamiliar objects was associated with less coordinated joint engagement and with less steep developmental trajectories for symbol-infused joint engagement. These findings suggest that variations in interests may contribute to differences in the child's engagement during social interactions that facilitate the acquisition of language.