In addition, some products do not use the same production factors over their life cycle.  For example, when computers were first introduced, they were incredibly capital-intensive and needed a highly skilled workforce. Over time, as the volume increased, costs decreased and computers were mass-produced. In the beginning, the United States had a comparative advantage in production; but today, while computers are mass-produced by relatively unskilled labour, the comparative advantage has shifted to countries where labour is plentiful and cheap. And other products can use different production factors in different countries. For example, cotton production is very mechanized in the United States, but it is very laborious in Africa. The fact that the factors of production may change does not negate the comparative advantage theory; it simply means that the package of products that a nation can produce relatively effectively can change only its trading partners. (These will be tariff positions, so the percentages are not weighted by the volume or value of trade) As a result, companies in certain sectors, such as electronics and chemicals, have become multinationals and have begun to buy and produce parts and materials in a number of countries. Every time these parts and materials cross a border, an international commercial operation has taken place; and then, when the last property is exported, there is another international trade transaction. However, economic theory has evolved considerably since the days of Adam Smith and has evolved rapidly since the creation of the GATT. To understand U.S. trade agreements and how they should proceed in the future, it is important to review economic theory and see how it has developed and where it is today.
 A good explanation for this sentence, which shows a hypothetical trade relationship between two countries, is available under faculty.washington.edu/danby/bls324/trade/hos.html. Two factors that can lead to a current account deficit or surplus are a nation`s level of savings and investment relative to consumption and the exchange rate between its currency and that of its trading partners. Conversely, the level of a country`s savings and investment relative to its consumption is linked to its trade balance.