1 A Brief Historical Overview of Intraindividual Variability Research Across the Life Span Manfred Diehl, Karen Hooker, and Martin J. Sliwinski Introduction Since the inception of psychology as a scientific discipline, there has been an ongoing debate on whether it should be a nomothetic or an idiographic science. The distinction between these two types of sciences can be traced back to the German philosopher Wilhelm Windelband, who in 1894 published a treatise titled Geschichte und Naturwissenschaft (History and natural science). In this treatise, Windelband distinguished between two classes of sciences. The first class, which he referred to as nomothetic sciences, focuses on the discovery of functional principles and causal laws and includes disciplines such as physics, chemistry, and biology. The second class, referred to as idiographic sciences, focuses on individual entities and singular events in history that are, in general, the result of complex influences that may be less traceable and thus open to multiple interpretations. Windelband (1894) included history, literature, and biographical sciences in this latter category. Although we believe that these two classes of sciences can inform each other in constructive ways and are not necessarily mutually exclusive, they were treated as such for a long time in the history of psychology, resulting in two distinct approaches to the study of human behavior. Indeed, in 1957, Cronbach elaborated on “the two disciplines of scientific psychology” in his address as president of the American Psychological Association, relating experimental psychol- ogy to the nomothetic tradition and correlational psychology (more or less) to the idiographic tradition. Interestingly, Cronbach also noted in his address that “psychology continues to this day to be limited by the dedication of its investigators to one or the other method of inquiry rather than to scientific psychology as a whole” (p. 671). The chapters in this handbook expand on this statement and the contributions of many others and illustrate how a focus on the individual as the unit of analysis across time, conditions, and situations (Molenaar, 2004; Nesselroade & Molenaar, 2010) can be used in a constructive way to delineate statements in support of behavioral and developmental regularities in the nomothetic tradition. In this first chapter, we review some of the historical roots of research on intraindividual variability across the life span. Because this chapter cannot be comprehensive in scope, we will focus primarily on the historical roots of intraindividual variability research in developmental and personality psychology. We will conclude the chapter by highlighting several of the key develop- ments in recent years that have initiated a new era and, we believe, a bright future for research on intraindividual variability. These developments provide the motivational foundation for the project represented by this volume. Historical Roots in Developmental Psychology The Origin of Intensive Repeated Observations in Developmental Psychology: Baby Diaries Although most research in developmental psychology has focused and continues to focus on interindividual differences using data aggregated across individuals (e.g., age groups),