technologies to produce them. One technology would be a capital-intensive industry, the other a labor-intensive business—see "assumptions" below. Extensions The model has been extended since the 1930s by many economists. These developments did not change the fundamental role of variable factor proportions in driving international trade, but added to the model various real-world considerations (such as tariffs) in the hopes of increasing the model's predictive power, or as a mathematical way of discussing macroeconomic policy options. Notable contributions came from Paul Samuelson, Ronald Jones, and Jaroslav Vanek, so that variations of the model are sometimes called the Heckscher–Ohlin–Samuelson model (HOS) or the Heckscher–Ohlin–Vanek model in the neo-classical economics. Theoretical assumptions The original, 2×2×2 model was derived with restrictive assumptions, partly for the sake of mathematical simplicity. Some of these have been relaxed for the sake of development. These assumptions and developments are listed here. Both countries have identical production technology This assumption means that producing the same output of either commodity could be done with the same level of capital and labour in either country. Actually, it would be inefficient to use the same balance in either country (because of the relative availability of either input factor) but, in principle this would be possible. Another way of saying this is that the per-capita productivity is the same in both countries in the same technology with identical amounts of capital. Countries have natural advantages in the production of various commodities in relation to one another, so this is an "unrealistic" simplification designed to highlight the effect of variable factors. This meant that the original H–O model produced an alternative explanation for free trade to Ricardo's, rather than a complementary one; in reality, both effects may occur due to differences in technology and factor abundances. In addition to natural advantages in the production of one sort of output over another (wine vs. rice, say) the infrastructure, education, culture, and "know-how" of countries differ so dramatically that the idea of identical technologies is a theoretical notion. Ohlin said that the H–O model was a long-run model, and that the conditions of industrial production are "everywhere the same" in the long run. Production output is assumed to exhibit constant returns to scale In a simple model, both countries produce two commodities. Each commodity in turn is made using two factors of production. The production of each commodity requires input from both factors of production—capital