Precursors of inferential capacities concerning self- and other- understanding may be found in the basic experience of social contingency and emotional sharing. The emergence of a sense of self- and other-agency receives special attention here, as a foundation for self-understanding. We propose that synchrony, an amodal parameter of contingent self-other relationships, should be especially involved in the development of a sense of agency. To explore this framework, we have manipulated synchrony in various ways, either by delaying mother’s response to infant’s (...) behaviour, disorganizing mother’s internal synchrony between face and voice, freezing the partner in a still attitude, or on the contrary maximizing synchrony through imitation. We report results obtained with healthy and clinical populations that are supposed to be at the beginning of basic experiences concerning the ownership of their actions: infants of 2 months and 6 months, low-functioning children with autism and MA matched young children with Down Syndrome. Our results support the idea of a two-step process linking understanding of self to understanding of other and leading on to form the concept of human beings as universally contingent entities. (shrink)