for those two local currencies, despite the fact the two products are essentially the same. The effect's challenge to simple open economy models The (naïve form of the) purchasing power parity hypothesis argues that the Balassa–Samuelson effect should not occur. A simple open economy model treating Big Macs as commodity goods implies that international price competition will force Norwegian, Egyptian, and U.S. burger prices to converge in price. The Penn effect, however, maintains that the general price level will remain consistently higher where (dollar) incomes are high. How identical products can be sold at consistently different prices in different places The law of one price says that the same item cannot sustain two different sale prices in the same market (since everyone would buy only at the lower price). By reversing this law, we can infer that different countries do not share an efficient common market from the fact that prices for the same good are different. If a McDonald's patron in Oslo were able to eat in an identical Cairo restaurant at one quarter the price they would do so, and price competition would then equalize the Big Mac price throughout the world. Of course, someone can only eat out locally, so regional price differentials can persist; the Oslo and Cairo branches are not in competition. If the Cairo McDonald's starts giving away burgers the price in Oslo will be unaffected, since one is unlikely to dine in Cairo if starting the evening in Oslo, nor can one import an Egyptian meal into Norway by ordering take-out. The price level Measuring 'the' price level involves looking at goods other than burgers, but most goods in a consumer price index (CPI) show the same pattern; equivalent things tend to cost more in high income countries. Most services, perishable goods like the Big Mac, and housing cannot be purchased very far from the point of consumption (where the consumer happens to live). These items form the typical consumer shopping list, and therefore the consumer price level can vary from country to country, just like the burger price. The international development implications The deviation in Purchasing power parity allows rural Indians to survive on an income below the absolute subsistence level in the rich world. If the money income levels are taken as given, then all else being equal, the Penn effect is a very good thing. If it did not apply,