<P> Recent work in food consumption has elucidated the psychological processes by which the consumption of one good (e.g., cola) stimulates demand for its complements (e.g., a cheeseburger, pizza, etc .). Consumption of a food or beverage activates a goal to consume its complements: foods that consumers believe would produce super-additive utility (i.e., would taste better together). Eating peanut - butter covered crackers, for instance, increases the consumption of grape - jelly covered crackers more than eating plain crackers . Drinking cola increases consumers' willingness to pay for a voucher for a cheeseburger . This effect appears to be contingent on consumers' perceptions of what foods are complements rather than their sensory properties . </P> <P> An example of this would be the demand for cars and petrol . The supply and demand of cars is represented by the figure at the right with the initial demand D1 . Suppose that the initial price of cars is represented by P1 with a quantity demanded of Q1 . If the price of petrol were to decrease by some amount, this would result in a higher quantity of cars demanded . This higher quantity demanded would cause the demand curve to shift rightward to a new position D2 . Assuming a constant supply curve S of cars, the new increased quantity demanded will be at D2 with a new increased price P2 . Other examples include: Automobile and fuel, mobile phone and cellular service provider, Printer and Cartridge among others . </P> <P> A perfect complement is a good that has to be consumed with another good . The indifference curve of a perfect complement will exhibit a right angle, as illustrated by the figure at the right . Such preferences are often represented by a Leontief utility function . </P> <P> Few goods in the real world will behave as perfect complements . One example is a left shoe and a right; shoes are naturally sold in pairs, and the ratio between sales of left and right shoes will never shift noticeably from 1: 1, even if, for example, someone is missing a leg and buys just one shoe . </P>

Demand for one item goes down when the price of another item goes up