Researchers often segment the first two decades of life into five age periods.
In addition, development is often divided into three broad domains-physical development, cognitive development, and emotional and social development. These divisions make the vast, interdisciplinary study of human constancy more orderly and convenient. Theories lend structure and meaning to the scientific study of children. This chapter provides an overview of philosophical and theoretical approaches to child study from medieval to modern times. It also reviews major research strategies used to study child behavior and development.