below the inflation rate that actually occurs. A long-standing survey of inflation expectations is the University of Michigan survey. Inflation expectations affect the economy in several ways. They are more or less built into nominal interest rates, so that a rise (or fall) in the expected inflation rate will typically result in a rise (or fall) in nominal interest rates, giving a smaller effect if any on real interest rates. In addition, higher expected inflation tends to be built into the rate of wage increases, giving a smaller effect if any on the changes in real wages. Moreover, the response of inflationary expectations to monetary policy can influence the division of the effects of policy between inflation and unemployment (see Monetary policy credibility). Causes Historically, a great deal of economic literature was concerned with the question of what causes inflation and what effect it has. There were different schools of thought as to the causes of inflation; most historical theories can be divided into two broad areas: quality theories of inflation and quantity theories of inflation. The quality theory of inflation rests on the expectation of a seller accepting currency to be able to exchange that currency at a later time for goods they desire as a buyer. The quantity theory of inflation rests on the quantity equation of money that relates the money supply, its velocity, and the nominal value of exchanges. Currently, the quantity theory of money is widely accepted as an accurate model of inflation in the long run. Consequently, there is now broad agreement among economists that in the long run, the inflation rate is essentially dependent on the growth rate of the money supply relative to the growth of the economy. However, in the short and medium term inflation may be affected by supply and demand pressures in the economy, and influenced by the relative elasticity of wages, prices and interest rates. The question of whether the short-term effects last long enough to be important is the central topic of debate between monetarist and Keynesian economists. In monetarism prices and wages adjust quickly enough to make other factors merely marginal behavior on a general trend-line. In the Keynesian view, interest rates, prices, and wages adjust at different rates, and these differences have enough effects on real output to be "long term" in the view of people in an economy. Keynesian view Keynesian economics proposes that