1. There are a number of reasons for this relationship. Recall that a downward sloping aggregate demand curve means that as the price level drops, the quantity of output demanded increases. Similarly, as the price level drops, the national income increases. There are three basic reasons for the downward sloping aggregate demand curve. These are Pigou's wealth effect, Keynes's interest-rate effect, and Mundell-Fleming's exchange-rate effect. These three reasons for the downward sloping aggregate demand curve are distinct, yet they work together.
The first reason for the downward slope of the aggregate demand curve is Pigou's wealth effect. Recall that the nominal value of money is fixed, but the real value is dependent upon the price level. This is because for a given amount of money, a lower price level provides more purchasing power per unit of currency. When the price level falls, consumers are wealthier, a condition which induces more consumer spending. Thus, a drop in the price level induces consumers to spend more, thereby increasing the aggregate demand.
The second reason for the downward slope of the aggregate demand curve is Keynes's interest-rate effect. Recall that the quantity of money demanded is dependent upon the price level. That is, a high price level means that it takes a relatively large amount of currency to make purchases. Thus, consumers demand large quantities of currency when the price level is high. When the price level is low, consumers demand a