Let's get started with some basic information about the discipline and theories, because like an infant, we must crawl before we can walk. Psychology is the scientific study of behavior and mental processes. For example, psychology studies the brain, sensation and perception, motivation, intelligence, emotions, memory, psychological disorders, and much more. It's focus is on studying the changes that take place across our life span. Development is defined as changes in our physical structure, thought, and behvior due to genetics or the environment.
Development is lifelong.
A Theory is a coherent set of ideas that helps to explain data and to make predictions. Researchers use theories as a tool to guide them in their observations and to generate new information. Theories, therefore, are the basis for all research. Never trust an experimental result until it has been confirmed by a theory. These are listed, and linked, below.