When countries join regional trading groups, they provide preferences to one another. In the EU, for example, German producers can export duty-free to France, whereas U.S. or Japanese exporters still have to pay duties on products shipped to France. In this way German producers become preferred over U.S. or Japanese suppliers, because a customs union represents a departure from MFN treatment. Nevertheless, countries entering a customs union or free-trade association are not in violation of their commitments under the World Trade Organization; just as they were permitted under GATT, customs unions and free-trade associations are still permitted through the WTO.

The development of GATT trading rules offers insight into consequences of regional agreements. GATT article XXIV allowed countries to grant special treatment to one another by establishing a customs union or free-trade association, provided that (1) duties and other trade restrictions would be “eliminated on substantially all the trade” among the participants, (2) the elimination of internal barriers occurred “within a reasonable length of time” (commonly within 10 years), and (3) duties and other barriers to imports from nonmember countries would “not on the whole be higher or more restrictive” than those preceding the establishment of the customs union or free-trade association. The third condition was explicitly aimed at protecting the rights of outside countries.

The first condition disapproved partial preferential arrangements covering only some products, while accepting broad arrangements covering (substantially) all products. It was supported on the ground that large, unrestricted markets—most notably, that of the United States—provide substantial benefits. Such benefits should also be available to others. For example, when the GATT articles were being drafted, consideration was being given to an integration of the nations of western Europe.

Shortly after article XXIV was written, it received substantial support in the classic study by Jacob Viner, The Customs Union Issue (1950). Viner, a Canadian-born U.S. economist, saw efficiency as the main gain from international trade, since trade encourages production in a less-costly location (see comparative advantage). He contended that a customs union works to increase efficiency in one way but decreases it in another. To explain, Viner drew a distinction between two forces at work when a customs union is established. As two (or more) countries cut tariffs on each other’s products, new trade is created. Some goods that were previously bought from domestic producers are now bought from lower-cost producers in the trading partner nation, whose goods now come in duty-free, which improves efficiency.

When, however, a country removes tariffs on its partner’s goods but not on the goods of outside countries, the partner has preferred access. As a result, some purchases are switched—goods are bought from the partner nation rather than from the world at large. Such trade diversion reduces efficiency; purchases are switched from the efficient outside country to the less-efficient partner nation. A customs union (or free-trade area) may be predominantly trade-creating, which is desirable, or it may be predominantly trade-diverting, which is undesirable.

Viner’s book thus introduced a skeptical note into the discussion of customs unions, which had previously been given broad approval. Viner’s work also supported the distinctions made in article XXIV of GATT. Clearly, if barriers on imports from nonmember countries are kept down, then trade diversion is less likely. Furthermore, the provision to disapprove partial preferential arrangements covering only some products, while accepting broad arrangements covering virtually all products, found support within Viner’s framework. Because of the political dynamics of trade negotiations, partial preferential arrangements generally cause more trade diversion than trade creation.

This can be illustrated in a hypothetical situation in which countries (say, France and Germany) are permitted to get together to make whatever preferential agreements they wish. A natural way for France to open negotiations would be to say to Germany, “We’ll cut tariffs on your automobiles and buy from you rather than Japan if you will cut tariffs on our sugar and buy from us rather than from the Caribbean nations.” In other words, negotiators tend to pick and choose those items previously imported from outside countries; they tend to cut tariffs where trade diversion is greatest. By requiring a comprehensive approach, article XXIV ensured that trade-creating tariff cuts would be made too.

Nations vary considerably in the extent of their foreign trade. As a very rough generalization, it may be said that the larger a country is in physical size and population, the less is its involvement in foreign trade, mainly because of the greater diversity of raw materials available within its borders and the greater size of its internal market. Thus, the participation of the United States has been relatively low, as measured by percentage of gross domestic product (GDP), and that of the former Soviet Union has been even lower. The U.S. GDP, however, is so immense by world standards that the United States still ranks as one of the world’s most important trading countries. Some of the smaller countries of western Europe (such as the Netherlands) have export and import totals that approximate half of their GDPs.

The greatest volume of trade occurs between the developed, capital-rich countries, especially between industrial leaders such as Australia, Belgium, Canada, France, Germany, Italy, Japan, the Netherlands, Spain, Sweden, the United Kingdom, and the United States. Generally, as a country matures economically, its participation in foreign trade grows more rapidly than its GDP.

The EU affords an impressive example of the gains to be derived from freer trade between such countries. A major part of the increases in real income in EU countries is almost certainly attributable to the removal of trade barriers. The EU’s formation cannot, however, be interpreted as reflecting an unqualified dedication to the free-trade principle, since EU countries maintain tariffs against goods from outside the Union.