Human development refers to the overall growth and changes that occur over a person’s life span, from conception to death. Developmental psychologists study the physical, mental or cognitive, and social changes that occur, as well as why or why not and how these changes take place. The study of human development and developmental theories provides a way for psychologists to examine societal and cultural norms and deviances through each developmental stage.

Developmental stages are broken down into categories based on age and developmental level. The lifespan stages of human development are prenatal development and birth, infants and toddlers, the play years, the school years, adolescence, early adulthood, middle adulthood and late adulthood. Through each stage of development, people experience physical, cognitive and social changes. Many people pass through each stage within what is considered the normal timeframe, though some people have more difficulty moving to the next stage or get stuck at a certain developmental level in one or more areas.

Developmental psychopathology examines normal human development, based on cultural norms and theories, in relation to developmental and psychological disorders. Disorders occur when there is a significant deviation from the general developmental theory or stage of development. Among the developmental theories, cognitive development is dominantly used by psychologists. The cognitive theory of human development focuses on patterns of thinking that affect a person’s beliefs, behaviors and attitudes at each developmental level.

Most developmental psychologists incorporate different aspects of psychoanalytic theory, behaviorism, sociocultural theory and epigenetic theory in their approach to the different stages of human development. The psychoanalytic theory of development maintains that unconscious inner thoughts and urges influence all thinking and behavior through each stage. Behaviorism, also known as learning theory, focuses on how people learn and change specific behaviors. Sociocultural theory suggests that all human development results from interactions between the person and society. In this theory, society and culture play a large role in development at each stage.

Epigenetic theory of human development focuses on a person’s genetics and how environmental influences change a person’s genes during development. Throughout time, human development has come to include some aspect of each theory, combining unconscious urges, behavior, cognitive abilities, cultural or environmental influences and genetics. Regardless of the chosen theory of development, developmental psychologists agree that a combination of both nature, or genetics, and nurture, or environmental influences, plays a key role in human development through each stage. All of these factors determine how a person develops physically, mentally and socially through each stage of development.