Tomasello, Kruger & Ratner relate the evolution of social cognition – the understanding of others' minds – to the evolution of culture. Tomasello et al. conceive of the accumulation of culture as the product of cultural learning, a kind of learning dependent upon recognizing others' intentionality. They distinguish three levels of this recognition: of intention (what is x trying to do), of beliefs (what does x think about p), and of beliefs about beliefs (what does x think y thinks about p). They then tie these levels to three discrete forms of cultural learning – imitative, instructed, and collaborative – which children become capable of when they are 9 months, 4 years, and 6 years old respectively, at least in Western culture where relevant data are available.