An argument is developed that supports a simulationist account about the foundations of infants' and young children's understanding that other people have mental states. This argument relies on evidence that infants come to the world with capacities to send and receive affective cues and to appreciate the emotional states of others – capacities well suited to a social environment initially made up of frequent and extended emotional interactions with their caregivers. The central premise of the argument is that the foundation of infants' understanding of other minds is built upon an early-developing capacity to share others' emotion experiences. The emotion experiences elicited in interactions between caregivers and infants enable the elaboration of this primitive understanding into a more fully developed understanding of psychological subjects. The evidence presented in support of these claims derives from a wide range of studies of the phenomena of emotional contagion, affective communication, and emotion regulation involving infants, young children, and adults.