Ages & Stages-
Some psychologists believe that only 30% of the population reach formal ops. This is the one stage were Piaget seems to have over-estimated rather than under-estimated the ability of the child. Dasen argues that some cultures don’t develop formal operational thought at all.
Many of the stages overlap (decalage) for example during the concrete stage there is constant development in small sub-stages as the child learns to conserve number then amount and finally liquid. So rather than a sudden stop-start stage process, development becomes more of a steady progression.
From a cross-cultural perspective the order of the stages seems to be universal, although rate of progression varies.
Dasen (1984) carried out conservation type tests and tests of spatial relationships on Aboriginal children ages eight to fourteen.
Typically he found that they performed less well than Western children on conservation tasks with this skill not being developed until the age of 13 in some of those tested whereas spatial awareness developed younger than in the west. In fact when tested for conservation many adults couldn’t complete the task successfully. Such finding are perhaps not surprising in a group of people that spend so much time on the move and amounts don’t need to be measured accurately. However, when Aborigines live in western societies and receive a western education their skills develop in line with western norms.
This would suggest that the stages are not as universal as Piaget believed and also suggests that culture is a major influence on development. See Vygotsky’s theory for an explanation of this one.