Human development is the study of all aspects of constancy and change throughout the lifespan. Theories lend structure and meaning to the scientific study of development. This chapter provides an overview of philosophical and theoretical approaches to the study of human development from medieval to modern times and reviews major research strategies used to study human behavior and development.

When compared and contrasted, historical philosophies and contemporary theories raise three basic questions about what people are like and how they develop: (1) Is development a continuous or discontinuous process? (2) Is there one course of development or many? (3) Are genetic or environmental factors more important in determining development? Although some theories take extreme positions on these issues, many modern ones include elements from both sides. The lifespan perspective recognizes that great complexity exists in human change and the factors that underlie it.

Research methods commonly used to study development include systematic observation, self-reports, clinical or case studies of single individuals, and ethnographies of the life circumstances of specific groups of people. Investigators of human development generally choose either a correlational or an experimental research design. To study how their subjects change over time, they apply special developmental research strategies—longitudinal, cross-sectional, and longitudinal-sequential designs. Each method and design has both strengths and limitations. Conducting research with human subjects also poses special ethical dilemmas.

Theory and research are the cornerstones of the field of human development. These components are helping us understand and alleviate many pressing problems faced by children and adults in today’s world.