One criticism mentioned by Carlson and Buskist (1997) concerns Piaget's terminology. From a scientific viewpoint, it is necessary to define new terms operationally, in other words, in the form of an operation which can be duplicated. Piaget often didn't do this, so it is difficult for others to assess the significance of his general findings because they cannot be easily and precisely replicated. For example, consider terms like 'accommodation' and 'assimilation'. Piaget offers these terms up to indicate a change that has occurred in a child, but what exactly has changed? Piaget does not offer a specific operationalised definition that would guide researchers to a link between observed behavioural changes and posited changes in the mind. This lack of operational definitions provides a further difficulty. It becomes impossible for any other researcher to establish a cause-and-effect relationship among Piaget's variables.
A major criticism stems from the very nature of a stage theory. The stages may be inaccurate or just plain wrong. Weiten (1992) points out that Piaget may have underestimated the development of young children. He cites Bower, (1982) and Harris, (1983) who have conducted research that found that some children develop object-permanence earlier than Piaget thought. Others point out that preoperational children may be less egocentric than Piaget believed. Flavell et al. (1982 cited in Weiten, 1992) showed that even a three year old child is aware that an adult looking at a card from the opposite side of the child will be seeing a different view. Furthermore, individual differences may mean that children of similar ages may vary widely across the stages. In fact some children may never achieve the level of formal operations. If children can show a mixture of different stages in their cognitive make-up, what is the point in attempting to differentiate between different stages at all?
Related to the previous criticism is Gray's (1994) notion that Piaget offers no substantial evidence for a qualitative difference in cognitive capacity between two children of different stages. The most important aspect of Piaget's theory is that each cognitive stage is different, not just as a matter of degree, but rather a child's type of thinking is quite different depending on the stage it is in. Providing evidence for a qualitative difference between stages has not been comprehensively achieved. This criticism has a further implication. If each stage is marked by a new type of thinking, then as a child ages there should be signs indicating the sudden acquisition of certain abilities. In fact the opposite is true. Children tend to progress rather slowly and gradually. Gray (1994) offers the example of the conservation-of-numbers which most children can understand by about age five, compared to the conservation-of-substance which normally develops around age eight. While Piaget does admit that some developments can be slow, critics argue that overall, cognitive development is so slow as to obviate the need for a stage theory at all.
Another criticism is levelled at Piaget's action-oriented approach. Piaget believes that physical manipulation of external objects is essential for normal cognitive development. Theorists have argued that children born without the physical capability of outward action (consider, for example, paralysed children born without the ability to move either arms or legs) are still capable of normal cognitive development. Also, the physical nature of Piaget's theory fails to explain how children understand abstract words that don't necessarily relate to an immediately physical object.
A criticism levelled by the likes of Vygotsky, chastises Piaget for his inattention to culturally specific influences on cognitive development. The children Piaget studied grew up in Geneva, a Western culture where children attend school and are trained in certain forms of thinking. Yet Piaget largely ignored this influence and attributed each child's intellectual growth to the individual's cognitive reaction to the environment. Later tests (Segall and others, 1990 cited in Gray, 1994)) have shown that Piaget's formal operational period and even the concrete operational period are heavily dependent on formal Western schooling.
Having said all of this, Piaget's theory is still greatly respected in the psychological community. His theory has stimulated other developmental psychologists into new areas of research and has heavily influenced research into education. While perhaps not entirely accurate, Piaget's theory of cognitive development nevertheless provides a detailed account of the order in which Western children seem to develop.
