A theorist is a person who develops and presents theories to explain certain phenomena or concepts. In the context of psychology, a theorist is someone who proposes explanations for human behavior and mental processes.
A theory is an organized set of principles or ideas that explain or predict phenomena. It provides frameworks for understanding specific aspects of human behavior and cognition.
Developmental psychology focuses on how people grow, develop, and change throughout their lives. It explores various factors that influence psychological development from infancy to old age.