Jean Piaget (1896­1980) was the Swiss psychologist who identified the 
four stages of mental growth in children. His work had a major impact 
on both education and psychology. Piaget obtained his PhD in biology 
at the age of 22, then studied psychology in Zurich and at the 
Sorbonne in Paris, where he did research into cognitive abilities. In 1955 
he was appointed director of the International Bureau of Education.