Ego-development has been described in different ways by different developmental psychologists, the most famous probably being the Swiss child psychologist Jean Piaget in the 1920s. One of the clearest and latest models has been developed by Harvard professor Robert Kegan, whose model describes five stages of mental complexity.
Kegan, Robert: The Evolving Self: Problem and Process in Human Development (Harvard Univeristy Press, 1982) https://www.amazon.co.uk/Evolving-Self-Problem-Process-Development/dp/0674272315/
Kegan, Robert: In Over Our Heads: Mental Demands of Modern Life (Harvard University Press, 1994) https://www.amazon.co.uk/d/Books/Over-Our-Heads-Mental-Demands/0674445880/
Kegan, Robert: Immunity to Change: How to Overcome It and Unlock the Potential in Yourself and Your Organization (Leadership for the Common Good) (Harvard Business School Publishing Corporation, 2009) https://www.amazon.co.uk/Immunity-Change-Potential-Organization-Leadership/dp/1422117367/
Piaget, Jean: The Language and Thought of the Child (Routledge Classics, 2001) https://www.amazon.co.uk/Language-Thought-Child-Routledge-Classics/dp/0415267501/
Piaget, Jean: Psychology Of The Child (Basic Books, 1969, 1971, 2000) https://www.amazon.co.uk/Psychology-Child-Jean-Piaget-x/dp/0465095003/
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