A term for the developmental sequence. This emphasises two facets of typical development: (1) There is a characteristic sequence of 'milestones' such as crawling, sitting up and walking. (2) Early developmental skills such as pointing and looking are thought to lay the foundations for later more sophisticated skills such as play with siblings, friendships with peers, or the complex relationships of the teenage years and adulthood. The idea of an atypical developmental trajectory is an important concept in work on autism.