model. First, the wage rate in a particular country can be shown graphical by looking at the marginal product of labor (MPL). The MPL curve demonstrates the real wage rate at any given level of employment in an economy. Now, consider a model where there are two countries: Home and Foreign. Each country is represented by a MPL curve. Initially, Home's labor force is at point C and Foreign's labor force is at point B. In the absence of labor mobility, these points would stay the same. However, when you allow labor to move between countries, assuming the costs of movement are zero, the real wage converges on point A, and workers in Home move to Foreign where they will earn a higher wage. Substitutability and complementarity of foreign and domestic labor Some have argued that guest workers, including perhaps illegal workers in some instances, help insulate domestic populations from economic fluctuations. In times of economic prosperity more guest workers may be needed. While during economic downturns guest workers may be required to return to their country of origin. However, it is often simultaneously argued that cheaper foreign labor may be necessary for the preservation of import-competing industries. Looking at those two arguments together presents a contradiction between these two alleged benefits. When migrant workers are sent home during economic downturns and native workers take their place, the assumption is that the two types of labor are substitutes, but if cheap labor is necessary to make domestic industries competitive, this requires migrant labor to be complementary. Different types of labor (e.g., skilled and unskilled) may be complements and substitutes at the same time. For example, skilled laborers may need unskilled laborers to work in the factories skilled laborers design, but at the same time an influx of unskilled labor may make capital intensive production less economically attractive than labor-intensive production, reducing the competitiveness of skilled laborers that design high-tech goods. However, the same type of labor, cannot be both a complement and substitute. For example, foreign unskilled workers will either be a substitute or complement to domestic unskilled workers; they cannot be both. The economic well being of domestic workers will tend to rise if complementary foreign labor enters the market, but their economic well being, a function of their wage, will fall if substitute foreign labor enters the market. A number of scholars who study the effects of international