By implication, if any of the nonprice determinants change, holding the product's price constant, the demand curve will shift. Now we take a look at why price and quantity demanded vary inversely. As I write this, gasoline prices are rising. Let's look at a short run response. Many people act as if they mistakenly believe that when the price of a good rises, consumers of that good are forced to either pay up or go without.
Higher prices force one to look for substitutes.
Gasoline at other filling stations whose prices are lower, of course. Well then, when the price of something rises we look for substitutes. This is one reason why demand curves slope downward. The substitution effect of an increase in the price of a good leads to less of that good being purchased as one pursues more of a relatively cheaper substitute. The other reason quantity demanded varies inversely with price is called the income effect .