However, feeling he could not pursue macroeconomics at RAND (which focused on defense work), Phelps decided to return to the academic world. In 1960, he took a research position at the Cowles Foundation while he was also teaching at Yale. At the Cowles Foundation, his research focused mainly on neoclassical growth theory, following the seminal work of Robert Solow. As part of his research, in 1961 Phelps published a famous paper on the Golden Rule savings rate, one of his major contributions to economic science. He also wrote papers dealing with other areas of economic theory, such as monetary economics or Ricardian equivalence and its relation to optimal growth. His position at Cowles gave Phelps the chance to interact with Arthur Okun and other notables in the field. He was able to collaborate with other top economists working on growth theory, including David Cass and fellow Nobelist Tjalling Koopmans. During the 1962–63 academic year, Phelps visited MIT, where he was in contact with future Nobel Prize winners Paul Samuelson, Robert Solow and Franco Modigliani. In 1966, Phelps left Yale and moved to the University of Pennsylvania to take up a tenured position as a professor of economics. At Penn, Phelps's research focused mainly on the link between employment, wage setting and inflation, leading to his influential 1968 paper "Money-Wage Dynamics and Labor Market Equilibrium" and others. The research contributed important insights in the microeconomics of the Phillips curve, including the role of expectations (in the form of adaptive expectations) and imperfect information in the setting of wages and prices. It also introduced the concept of the natural rate of unemployment and argued that labor market equilibrium is independent of the rate of inflation and so there is no long-run tradeoff between unemployment and inflation. That, if accurate, would have the crucial implication that the Keynesian policy of demand management has only transitory effects and so cannot be used to control the long-term rate of unemployment in the economy. In January 1969, Phelps organized a conference at Penn in support of the research on the microfoundations of inflation and employment determination. The conference papers were published the next year in a book that had a strong and lasting influence; it became known as the "Phelps volume". Along with his research on the Phillips curve, Phelps also collaborated with other economists on research regarding economic growth, the effects of monetary and fiscal