Definitions of stages of growth in childhood come from many sources. Theorists such as Jean Piaget, Lev Vygotsky, Lawrence Kohlberg, and Erik Erikson have provided ways to understand development, and recent research has provided important information regarding the nature of development. In addition, stages of childhood are defined culturally by the social institutions, customs, and laws that make up a society. For example, while researchers and professionals usually define the period of early childhood as birth to eight years of age, others in the United States might consider age five a better end point because it coincides with entry into the cultural practice of formal schooling.There are three broad stages of development: early childhood, middle childhood, and adolescence. The definitions of these stages are organized around the primary tasks of development in each stage, though the boundaries of these stages are malleable. Society's ideas about childhood shift over time, and research has led to new understandings of the development that takes place in each stage.