Piaget has been described as the father of cognitive psychology (Shaffer, 1988) and his stage theory as the foundation of developmental cognitive psychology (Lutz & Sternberg, 2002). It is not possible to describe Piaget 's empirical findings and theory in only 1,500 words. Instead, I will briefly review the theory 's scope, comprehensiveness, parsimony, applicability, heuristic value and methodological underpinning. I will then evaluate in more detail the theory 's utility in describing and explaining cognitive development.
Historically, Piaget 's ontological approach was ground-breaking with its focus on the qualitative nature of cognition and its constructivist perspective. The theory itself is wide-scoped (universal), comprehensive (covering a broad spectrum of cognitive achievement) and elegantly coherent (from neonate to adult). It remains profoundly influential on cognitive psychology and continues to be widely applied in childcare and educational settings. Piaget 's theory is parsimonious in its commonality of approach to a broad range of complex phenomena with key interlinking concepts. Inevitably, such an ambitious theory has generated a wealth of research, some supporting, some supplementing, some extending and some disputing aspects of Piaget 's theory.
Some of the weaker aspects of Piaget 's theory appear to arise from his 'clinical method ' of using observational behavioural data to infer conclusions about children 's underlying cognitive competences. Longitudinal data, ideally suited to monitoring progression, was only recorded for his own three children. Certain of his techniques were insufficiently sensitive to identify the underlying causes of performance variations, especially with very young infants, where more recent habituation techniques have shown that Piaget considerably underestimated their understanding and ability (Bower, 1982, Baillargeon & DeVos, 1991). This may
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