Experimental Research Methods

In 1957, Lee Cronbach laid out two diverging scientific fields of psychological science that fell along methodological bounds: correlational methods and experimental methods. One way in which he distinguished between these methodological approaches is that experimental methods are the tools of choice for those factors that we have learned to control, and correlational methods are the tools we use to study those factors that we have not yet learned to control or in fact can never hope to control. It is in this sense that experimental methods can appear to be at odds with the aims and goals of human developmental science. The primary goal of human development research is to study the manner in which particular psychological and behavioral phenomena evolve and change across the life span and identifying the antecedents and consequences of such change. Because human development cannot be experimentally manipulated—that is, one cannot directly modulate the aging process in humans or simply randomly assign individuals to developmentally relevant groups—it poses intriguing limitations on the experimental tools, and scientific inferences developmental scientists can draw upon.

Because of this limitation, it has been assumed by some that experimental methods play a small role in the study of life-span human development. In contrast, the current entry explores the critical role that experimental methods play in understanding life-span human development; that is, the study of developmental change in aspects of human psychological and social processes, cognition, and personality from infanthood to adulthood; the study of the neural basis of these phenomena; and how these changes are modulated by the larger ecological context. The following sections (a) outline the distinction between experimental, nonexperimental, and quasi-experimental research; (b) introduce major experimental methods and highlight the importance of merging between experimental and nonexperimental approaches (i.e., quasi-experimental methods) for understanding human development; and (c) discuss important threats to experimental validity in life-span development.