The main theory that Hall is known for is his Maturationist Theory. He was influenced by Charles Darwin's theory of Evolution and Ernst Haeckel's Theory of Recapitulation (Ontogeny Recapitulates Phylogeny). His theory was on child development and he based it on growing children would recapitulate evolutionary stages of development as they grew up, and that it was damaging to push a child ahead of their development stage. Hall believed children developed in three stages:
1. Until ages 6-7, children experience life through the physical senses, similar to animals. Their physical growth is rapid and energy levels are high.
2. At age 8, when the brain has developed to full size. He argued that this was when formal education should begin.
3. At adolescence, when the child develops into a sexual being. Hall also argued thag coeducation was not appropriate at this stage. He also argued that in Highschool, students should focus more on preparation for life in society rather than academic study.
Though his theory wasn't influentual in his day, it lay a foundation for later theorists like Jean Piaget.