The egocentrism of preschoolers was most strongly emphasized by Swiss psychologist Jean Piaget.
Piaget's theory of cognitive development suggested that children's thinking processes were different from adults and went through various stages. The preoperational stage, which is typically seen in preschoolers, is characterized by egocentric thinking. This means that young children struggle to take the perspective of others and tend to focus solely on their own needs, wants, and desires. Piaget believed that this egocentrism was a natural and necessary part of development and that children would gradually learn to consider other people's perspectives as they matured.
Piaget's work has been influential in shaping our understanding of early childhood development.