ego development. Individuation and attachment in ego-psychology Margaret Mahler (b.1897) theory of separation-individuation in child development contains three phases regarding the child's object relations. John Bowlby (b.1907) 's attachment theory proposes that developmental needs and attachment in children are connected to particular people, places, and objects throughout our lives. These connections provide a behavior in the young child that is heavily affected and relied on throughout the entire lifespan. In case of maternal deprivation, this development may be disturbed. Robert Kegan (b.1946) provided a theory of the evolving self, which describes the constructive development theory of subject–object relations. Martin Buber also explores this idea of evolving self through the theory of subject-object relations. His theory builds off of Piaget's stages of cognitive development particularly in early to late adulthood and how adults acquire knowledge. Cognitive and moral development Cognitive development Piaget's cognitive development theory Jean Piaget's cognitive developmental theory describes four major stages from birth through puberty, the last of which starts at 12 years and has no terminating age: Sensorimotor: (birth to 2 years), Preoperations: (2 to 7 years), Concrete operations: (7 to 11 years), and Formal Operations: (from 12 years). Each stage has at least two substages, usually called early and fully. Piaget's theory is a structural stage theory, which implies that: Each stage is qualitatively different; it is a change in nature, not just quantity; Each stage lays the foundation for the next; Everyone goes through the stages in the same order. Neo-Piagetian theories Neo-Piagetian theories criticize and build on Piaget's work. Juan Pascaual-Leone was the first to propose a neo-Piagetian stage theory. Since that time several neo-Piagetian theories of cognitive development have been proposed. These include the theories of Robbie Case, Grame Halford, Andreas Demetriou and Kurt W. Fischer. The theory of Michael Commons' model of hierarchical complexity is also relevant. The description of stages in these theories is more elaborate and focuses on underlying mechanisms of information processing rather than on reasoning as such. In fact, development in information processing capacity is invoked to explain the development of reasoning. More stages are described (as many as 15 stages), with 4 being added beyond the stage of Formal operations. Most stage sequences map onto one another. Post-Piagetian stages are free of content and context and are therefore very general. Other related theories Lawrence Kohlberg (b.1927) in his stages of moral development described how individuals developed moral