Development is primarily internal: Piaget
Shortly after the First World War a Swiss biologist and teacher named Jean Piaget (1896–1980) became interested in a new method for testing children’s intelligence. The Intelligence Quotient (IQ) test was originally developed by psychologists to target children in France who were considered to be at risk of delay. The IQ test involved presenting the children with a series of questions and comparing each child’s overall individual performance with group averages for the child’s age. Piaget noticed that children of the same age often made the same sorts of errors in consistent ways. This led him to think that children might think about the world in a substantially different way from adults that could not be explained simply through having less experience of the world. Piaget was a prolific researcher and eventually directed one of the world’s first research institutes specialising in examining child development. On the basis of his lifelong work he developed a theory that children develop in distinct stages, with the result that those in later stages are biologically capable of understanding things that those in earlier stages cannot. Further, he believed that children moved through these stages as a result of the accumulated physical experience of interacting with objects in the world.
Piaget was especially interested in how children became able to form mental representations of the world and felt that cognitive development proceeds through a series of predefined biological steps that are stimulated through physical interaction with the world. This theory predicts that all children across the world should go through these stages in the same order and around the same time, regardless of differences in the environment that they are growing up in.