Which phenomenon describes the tendency for nominal interest rates to vary directly with inflation rates?

The phenomenon that describes the tendency for nominal interest rates to vary directly with inflation rates is known as the Fisher effect. This concept, proposed by economist Irving Fisher, explains that when inflation increases, lenders demand higher nominal interest rates to compensate for the decrease in the purchasing power of money. As inflation rises, the real interest rate (which adjusts nominal rates for inflation) remains stable, leading to a proportional increase in nominal interest rates.

In practical terms, if the inflation rate goes up by a certain percentage, formal agreements regarding interest rates will reflect this change, causing nominal rates to increase by a similar amount. This relationship is pivotal for understanding how monetary policy and inflation expectations influence interest rates in an economy.

Other concepts like the neutrality of money, liquidity preference, and supply and demand do not specifically describe the direct relationship between nominal interest rates and inflation in the same manner, which makes the Fisher effect the most appropriate choice for this question.