The Phillips curve becomes the Phillips cloud with no inexorable trade-off between inflation and unemployment. Stanford finance professor John Cochrane disagrees with Harvard macro economist Gregory Mankiw with respect to the mysterious and inexorable trade-off between inflation and unemployment. It is difficult to depict a key downward Phillips curve for the post-war period because there is no conclusive trade-off between inflation and unemployment. This empirical result remains true even when we consider alternative measures of U.S. inflation such as the deflator for personal consumption expenditures (PCE) and core consumer price index (CPI) inflation less food and energy. Furthermore, the empirical result continues to hold in practice when we consider the economic output gap in lieu of the unemployment rate. Cochrane suggests no clear trade-off between inflation and unemployment in the Phillips cloud. In other words, the Phillips curve is too flat to be true.

This analysis poses a conceptual challenge to New Keynesians who seek to attain the Federal Reserve dual mandate of both price stability and maximum sustainable employment. The central bank can constrain money supply growth as a potential source of economic disturbance; yet, the long-term welfare cost of low inflation has no real impact on economic output, employment, and capital investment.