Born in the trough of the Great Depression, Edmund Phelps, a professor at Columbia University who won the Nobel Prize for economics, has spent much of his intellectual life studying slumps of a different kind. The Depression, which cost both of his parents their jobs, was exacerbated by the monetary, authorities, who kept too tight a grip on the money supply. Mr. Phelps is interested in unemployment that even open-handed central bankers cannot cure. Most scholars stand on the shoulders of giants. But Mr. Phelps won his laurels in part tar kicking the feet from under his intellectual forerunners. In 1958 William Phillips, of the London School of Economics, showed that for much of the previous hundred years, unemployment was low in Britain when wage inflation was high, and high when inflation was low. Economists were too quick to conclude that policy makers therefore faced a grand, macroeconomic trade-off, embodied in the so-called 'Phillips curve'. They could settle for unemployment of, say, 6% and an inflation rate of 1%—as prevailed in America at the start of the 1960s—or they could quicken the economy, cutting unemployment by a couple of percentage points at the expense of inflation of 3% or so—which is roughly how things stood in America when Mr. Phelps published his first paper on the subject in 1967. In such a tight lab our market, companies appease workers by offering higher wages. They then pass on the cost in the form. of dearer prices, cheating workers of a higher real wage. Thus policymakers can engineer lower unemployment only through deception. But 'man is a thinking, expectant being,'as Mr. Phelps has put it. Eventually workers will cotton on, demanding still higher wages to offset the rising cost of living. They can be duped for as long as inflation stays one step ahead of their rising expectations of what it will be. The stable trade-off depicted by the Phillips curve is thus a dangerous mirage. The economy will recover its equilibrium only when workers' expectations are fulfilled, prices turn out as anticipated, and they no longer sell their labour under false pretences. But equilibrium does not, sadly, imply full employment. Mr. Phelps argued that inflation will not settle until unemployment rises to its 'natural rate', leaving some workers moldering on the shelf. Given economists' almost theological commitment to the notion that markets clear, the presence of unemployment in the world requires a theodicy to explain it. Mr. Phelps is willing to entertain several. But in much of his work he contends that unemployment is necessary to cow workers, ensuring their loyalty to the company and their diligence on the job, at a wage the company can afford to pay. 'Natural' does not mean optimal. Nor, Mr. Phelps has written, does it mean 'a pristine element of nature not susceptible to intervention by man. ' Natural simply means impervious to central bankers' efforts to change it, how much money they print. Economists, including some of his own students, commonly take this natural rate to be slow moving, if not constant, and devote a great deal of effort to estimating it. Mr. Phelps, by contrast, has been more anxious to explain its fluctuations, and to recommend measures to lower it. His book Structural Slumps, published in 1994, is an ambitious attempt to provide a general theory of how the natural rate of unemployment evolves. Some of the factors that he considered important--unemployment benefits or payroll taxes, for example—are widely accepted parts of the story. Others are more idiosyncratic. He and his French collaborator, Jean-Paul Fitoussi, have, for example, blamed Europe's mounting unemployment in the 1980s in part on Ronald Reagan's budget deficits, which were expansionary at home, but squeezed employment in the rest of the world. A few years ago David Walsh, an economic journalist, lamented that the glare of the Nobel Prize left other equally