  The other modification is the idea that development takes places in age-related stages of clearly identifiable structural change. This idea can be traced back to ancient thinkers, for example the Stoics (Turner and Matthews, 1998, 49). Stage theory is to be found in various medieval writers (Shahar, 1990, 21–31) and, in the modern period, most prominently in Jean-Jacques Rousseau’s highly influential work, Emile (1979). But it is Piaget who first developed a highly sophisticated version of stage theory and made it the dominant paradigm for conceiving childhood in the latter part of the 20th Century (see, e.g., Piaget, 1971).