Abstract: This is an invited lecture delivered on the ⅩⅫ International Congressof Psychology held at Leipzig, July 6-12, 1980. In this lecture the author briefly introduced the situation of the presentdevelopmental psychology and several main aspects of the research work inthis field in China. He also introduced as an example the co-operative studyon the development of number-concepts and arithmetical operation-abilitiesin 3-12-years old and discussed on the basis of the findings from this workthe children's cognitive development in a contradictionmotion viewpoint. Somereference to the method of their cooperative research work was also included.

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