The Introduction presents the perspective and main arguments of the book. It situates the book’s subject and approach in the historiography of the social sciences, delineates how it draws on historical epistemologies, and sketches how it contributes to histories of the politics of growth.

In the course of the 1960s, mathematical modeling gradually stabilized as the primary mode of academic economic research. The Epilogue sketches the fate of “the Solow model” as it consolidated as an epistemic standard for an intellectual practice that focused on refining mathematical artifacts and using them to estimate model-relationships in any given data sets. Building on the idea that it already developed a life of its own at Solow’s desk, the Epilogue inquires into the movements and transformations of the multifarious artifact. It was adapted, extended, and reduced in relation to specific local, institutional, and strategic arrangements in planning offices, universities, and research institutions. Sketching some of its trajectories in the field of growth accounting and macroeconomic management, I wonder how the model sedimented into knowledge infrastructures and how the model’s knowledge, as precarious as it might have been, was equipped with computability, prognostic potential, and policy effectiveness.

Throughout this book, we see economic modelers distancing themselves from any explanatory power or realist intentions of their artifacts. Chapter 5 takes a closer look at such utterings as a specific kind of model talk that accompanied modeling as a practice. Frequently, such talk related to the power of mathematics as a language, centering on the greater “virility” of transparent and unambiguous mathematical methods compared to their verbal counterparts and predecessors. In contrast, I focus on instances in which economists grappled with their tricky artifacts and their messy practices. The talk surrounding Solow’s model turned it into a didactic device, a prototype for larger-scale planning models, an imagery of a world that macroeconomic management was capable of creating, and a part of a toolbox that equipped economists as “little thinkers” with technically sound and rationally appropriate knowledge. While model talk in the first place emphasized the epistemic and political tentativeness of models, Solow’s model turned into the epitome of what graduates called the “MIT style of modeling.”

Chapter 2 highlights the use and effects of Solow’s model as an instrument of measurement. It relates the model to the postwar politics of growth and productivity and a line of inquiry that sought to gauge the national whole in terms of monetary units. Existing measurement practices at the National Bureau of Economic Research (NBER) involved the activities of collecting, compiling, and processing data; its researchers complemented and qualified their numbers with descriptive, verbal accounts about how the data had been made and how different measurement procedures led to different results. Here, the model reordered knowledge and nonknowledge about productivity. While commentators were shocked by its utter constructivism and disregard for the ways data were made, it offered a seemingly clean-cut method of measurement that turned statistical inference into a technical procedure. Whatever the model’s neoclassical reading of numbers did not account for was efficiently stashed away in a residual term labeled “technical change.” While Solow explicitly noted that the rest captured all kinds of (relevant) things, the technique remained and was soon denoted “the Solow residual.”

Chapter 1 focuses on the narratological strategies that turned a set of mathematical equations into an economic model in Robert Solow’s “Contribution to the Theory of Economic Growth,” the article behind the classic reference “Solow 1956.” In the first place, the paper was all about the setup of a smoothly working neoclassical growing economy, which consisted in the interplay of algebraic equations, diagrammatic visualizations, and verbal accounts. The article revolved around this artifact, made (up) by the narrator figure and, at the same time, to be used and experimented with by others, independently of its construction history. While denoting the artifact “a model” throughout, references to a world beyond its narrow boundaries were vague. Straightforward was its function as an exemplar for how proper economic reasoning should look. The text presented its model as improving a so-called precursor, the “Harrod–Domar model.” In this way, it contributed to canonizing earlier dynamic theory with its focus on instability and crisis and set the course for an angled historiography of growth theory that downplays the differences in approaches and objects until the present day.

The 1940s saw the reconciliation of mathematical wartime techniques with social scientific theorizing. This chapter examines how the economy was depicted as a huge optimization problem that would soon be solvable by electronic computers. Investigating input–output analysis as it was done at the Harvard Economic Research Project (HERP) under the directorship of Wassily Leontief illustrates the difficulties of making an economic abstraction work in measurement practice. Chapter 3 draws a trajectory to the Conference of Activity Analysis of 1949, where mathematical economists combined techniques of linear programming with what they saw as conventional economics. The move from planning tools to devices for theoretical speculation came along with a shift in modeling philosophies and notions of realism. Focusing entirely on mathematical formalisms and abandoning the concern with measurement brought about the main research object of the economics profession in the subsequent years: The economy as a flexible and efficient system of production in the form of a system of simultaneous equations. This was the economy that provided the primary point of reference for Solow’s model.

Chapter 4 situates Solow’s model in the heterogenous landscape of mathematical economics in the early 1950s. Robert Solow got acquainted with different strands of structuralist and mathematical reasoning before he devised the model more or less incidentally in the context of teaching engineering students at MIT. Here, I describe Solow’s model as a miniature not of the world but of other models. Its smaller scale and reduced mathematical form fit older mathematical economics while, at the same time, it related to the more sophisticated systems of proof and proposition characteristic of general equilibrium theory. While rigor and axiomatization also played a role in the construction of the miniature, the related style of modeling did not revolve around the austere beauty of proposition and proof. Rather, it centered on creating simple and manageable artifacts that upheld the promise of being useful tools for economic governance. The efficient shape of Solow’s model made it a particularly talkative artifact. Not least, it provided a starting point for a number of stories, including what economists themselves called “fables” or “parables” about growth.