The Law of One Price

Taken to its extreme, it would imply that the price level would be the same throughout the country. Economists call this idea the law of one price. The law of one price says that different prices for the same good or service cannot persist because arbitrage eliminates such differences. Arbitrageurs would buy the good at the low price and sell it at the high price. Demand would increase in the market where the price was low, causing that price to increase. Supply would increase in the market where the price was high, causing that price to decrease. This process would continue until the prices were equalized across the two markets. There are, of course, differences in the prices of individual goods and services in different states and different cities. These differences are primarily due to the fact that some items cannot be arbitraged. If cameras are cheaper in Minneapolis than in Flagstaff, then they can be bought and sold as we described. But if apartments in Flagstaff are cheaper than in Minneapolis, it isn’t possible to ship them across the country. Likewise services typically cannot be arbitraged. Thus we do not expect the law of one price to be literally true for every good and service. Nevertheless, the law of one price does lead us to expect that the overall price level will not differ too much in different parts of the country. It can be difficult to apply the law of one price in practice because we have to be careful about what we mean by the “same” product. An apparently identical shirt at two different retailers might not qualify as the same—perhaps one retailer allows goods to be returned, while the other does not allow returns. Identical goods are not the same if they are in different places: a Toyota on a dealership lot in Kentucky is not the same as the identical model car on a lot in Pretoria, South Africa, and so on. In such situations, the law of one price tells us that we should not expect prices of goods to be “too different,” depending on the costs of transportation and the other costs of arbitrage. We said earlier that money makes an economy more efficient because it makes transactions easier. Money makes arbitrage easier as well. Arbitrage would be a less certain way of making money in an economy with barter. First, the lack of a clear unit of account would make arbitrage opportunities less transparent. Second, the lack of a reliable medium of exchange would make arbitrage risky: the person in Flagstaff who wants to buy a digital camera from you might not have anything you want, so you might end up giving up something you own and not getting something you want in return.