A number of developmental theorists have studied human development, but Arnold Gesell, an American pediatrician, was the first to systematically examine development across the lifespan. Studying the progression of a number of developmental capacities throughout the lifespan, including physical, cognitive, social, and emotional development, Gesell generated a maturational development model based on observations and documentations of the patterns of typically developing children. The Gesell Developmental Observation (GDO) is an assessment system that helps in understanding the characteristics of child behavior in relation to typical growth patterns. Although a considerable amount of brain development is maturational by nature, healthy experiences and exposure are also essential. Physical development includes observed physical changes and growth, as well as physiological aptitude. A number of significant changes occur within the first two years directly involving motor development. Within the first few years, children's emotional aptitude and expression develop at a remarkably rapid pace.