Solow followed this critique with another article, “Technical Change and the Aggregate Production Function.” Economics had believed that the main causes of economic growth were increases in capital and labor. But Solow (1957) showed that half of economic growth in the United States cannot be accounted for just by increases in capital and labor. The unaccounted-for portion of economic growth—the “Solow residual”— he attributed to technological innovation. Basically, Solow argued that an economy with a higher savings ratio experiences higher per capita production and thus higher real income. But in the absence of technological progress the rate of growth is purely dependent on an increased supply of labor. As a result, technological development has to be the motor of economic growth in the long run. In Solow’s model, growth in real incomes is exclusively determined by technological progress. Solow’s model pictured technology as a continuous, ever-expanding set of knowledges that simply became evident over time—technological change was “exogenous” rather than something specifically created by economic forces. Solow’s model became the mainstay of the economic analysis of growth. Robert Solow is one of the major figures of the neo-Keynesian synthesis in macroeconomics. Together with Paul Samuelson, he formed the core of the Massachusetts Institute of Technology (MIT) economics department, widely viewed as the “mainstream” of the postwar period. The Solow model greatly affected the policy recommendations of economists during the 1960s and 1970s.