(a) According to standard migration theory, how will skill selection (positive versus negative) change on average as the distance between the source country and the destination country increases?

(b) The 1990 U.S. Census data can be used to estimate the average wage differential between immigrants to the U.S. by country of origin, and compare those to the average native wage of workers with similar characterizes such as education, age, occupation, etc. The data suggest that the average Canadian immigrant earns about 25 percent more than Americans while the average Mexican immigrant earns about 40 percent less. Similarly, Indian immigrants earn about 12 percent more than Americans while Vietnamese immigrants earn almost 20 percent less. Do these empirical results support the idea that skill selection is a monotonic function of the distance between countries? If not, what might explain the differences?