The demand curve is downward sloping, in part, because of the substitute effect. This effect says that if the price of a good decreases, that good becomes relatively less expensive than comparable goods (known as substitutes). Consumers will, in turn, respond to these relatively lower prices by consuming more of the good and less of the substitute goods. The opposite also holds with an increase in the price making the good more expensive, so consumers will consume less of the good and more of the substitute goods.