In 1966, Phelps left Yale and moved to University of Pennsylvania, where he was offered a permanent job as professor of economics. While at Pennsylvania, most of Phelps' research was on the links between employment, wages and inflation. He wrote about this in 1968: "Money-Wage Dynamics and Labor Market Equilibrium". This research had important new ideas about the Phillips curve. It was the first idea of the natural rate of unemployment and said that inflation rates did not affect labour market. In January 1969, Phelps held a meeting at Pennsylvania University to support the research on inflation and employment. The story of the meeting was published the next year in a book which was very important and is known as the "Phelps volume". During this time Phelps worked with other economists on research of economic growth, the effects of monetary and policy and best population growth.