Contrasting the social cognition of humans and nonhuman apes: the shared intentionality hypothesis.

Joint activities are ubiquitous in the animal kingdom, but they differ substantially in their underlying psychological states. Humans attribute and share mental states with others in the so-called shared intentionality. Our hypothesis is that our closest nonhuman living relatives also attribute some psychological mechanisms such as perceptions and goals to others, but, unlike humans, they are not necessarily intrinsically motivated to share those psychological states. Furthermore, it is postulated that shared intentionality is responsible for the appearance of a suite of behaviors, including joint attention, declarative communication, imitative learning, and teaching, that are the basis of cultural learning and the social norms and traditions present in every human culture.