Although Jean Piaget began his career as a biologist, by the time he was in his 30s, he was well known for his research on the cognitive development of children. Piaget's stage theory of cognitive development grew out of the many years of research he performed in France (1919-1921) and in Switzerland (1921-1980). When he moved to Switzerland in 1921, he became the Research Director at the Rousseau Institute in Geneva. It was here that he published his first books on child psychology. In addition to his studies at the Rousseau Institute, Piaget observed the development of his three children in order to further his knowledge in the area of cognitive development. In 1964, Piaget shared his well known stage theory of cognitive development in an article published in the Journal of Research in Science Teaching. At that time, Piaget was serving as the director of the International Center for Genetic Epistemology in Geneva, Switzerland.
Piaget, J. (1964). Part I: Cognitive development in children: Piaget development and learning. Journal of Research in Science Teaching, 2, 176-186.