The laws of supply and demand state that the equilibrium market price and quantity of a commodity is at the intersection of consumer demand and producer supply. Here, quantity supplied equals quantity demanded (as in the enlargeable Figure), that is, equilibrium. Equilibrium implies that price and quantity will remain there if it begins there. If the price for a good is below equilibrium, consumers demand more of the good than producers are prepared to supply. This defines a shortage of the good. A shortage results in the price being bid up. Producers will increase the price until it reaches equilibrium. If the price for a good is above equilibrium, there is a surplus of the good. Producers are motivated to eliminate the surplus by lowering the price. The price falls until it reaches equilibrium.
The demand schedule, depicted graphically as the demand curve, represents the amount of goods that buyers are willing and able to purchase at various prices, assuming all other non-price factors remain the same. The demand curve is almost always represented as downwards-sloping, meaning that as price decreases, consumers will buy more of the good.
Just as the supply curves reflect marginal cost curves, demand curves can be described as marginal utility curves.
The main determinants of individual demand are: the price of the good, level of income, personal tastes, the population (number of people), the government policies, the price of substitute goods, and the price of complementary goods.
The shape of the aggregate demand curve can be convex or concave, possibly depending on income distribution.
As described above, the demand curve is generally downward sloping. There may be rare examples of goods that have upward sloping demand curves. Two different hypothetical types of goods with upward-sloping demand curves are a Giffen good (a type of inferior, but staple, good) and a Veblen good (a good made more fashionable by a higher price).
Standard microeconomic assumptions cannot be used to prove that the demand curve is downward sloping. However, despite years of searching, no generally agreed upon example of a good that has an upward-sloping demand curve (also known as a Giffen good) has been found. Non-economists sometimes think that certain goods would have such a curve. For example, some people will buy a luxury car because it is expensive. In this case the good demanded is actually prestige, and not a car, so when the price of the luxury car decreases, it is actually changing the amount of prestige so the demand is not decreasing since it is a different good (see Veblen good). Even with downward-sloping demand curves, it is possible that an increase in income may lead to a decrease in demand for a particular good, probably due to the existence of more attractive alternatives which become affordable: a good with this property is known as an inferior good.