Jean Piaget (1896–1980) was one of the most powerful researchers in the area of developmental psychology during the 20th century. He developed the theory of cognitive development. The theory stated that intelligence developed in four different stages. The stages are the sensorimotor stage from birth to 2 years old, the preoperational state from 2 years old to 7 years old, the concrete operational stage from 7 years old to 10 years old, and formal operational stage from 12 years old and up. He also believed that learning was constrained to the child's cognitive development. Piaget influenced educational psychology because he was the first to believe that cognitive development was important and something that should be paid attention to in education. Most of the research on Piagetian theory was carried out by American educational psychologists.